[android] print backtraces on ds2 crashes/errors

## Purpose
Add support to print ds2 backtraces on Android from the signal handler
to diagnose crashes and fatal errors in DS2 itself.

## Overview
* Reuse the existing backtrace plumbing and implement only the glibc
`backtrace` functionality missing on Android
* Log the module-relative address of each stack frame in addition to the
existing information.

## Problem Details
No backtraces are currently logged when DS2 crashes or encounters a
fatal error on Android because `ds2::utils::PrintBacktrace` function is
stubbed-out on platforms without glibc. Instead of glibc, Android
provides `libunwind` in the NDK exposed in the `unwind.h` header.

## Validation
Manually verified mostly complete backtraces are emitted from signal
handler context during a ds2 crash on a Debug. Verified with ds2 built
for 4 Android ABIs. x86, x86_64, armeabi-v7a, and arm64-v8a.

Example fatal error log (from x86_64):
```
[10853][void ds2::Utils::PrintBacktrace()] ERROR  : 0x00005f03b2caed35 <unknown>+0x5f03b2caed35 (/data/local/tmp/ds2+0xa7d35)
[10853][void ds2::Utils::PrintBacktrace()] ERROR  : 0x00005f03b2cafb88 <unknown>+0x5f03b2cafb88 (/data/local/tmp/ds2+0xa8b88)
[10853][void ds2::Utils::PrintBacktrace()] ERROR  : 0x00005f03b2cc144f <unknown>+0x5f03b2cc144f (/data/local/tmp/ds2+0xba44f)
[10853][void ds2::Utils::PrintBacktrace()] ERROR  : 0x00005f03b2c785ed <unknown>+0x5f03b2c785ed (/data/local/tmp/ds2+0x715ed)
[10853][void ds2::Utils::PrintBacktrace()] ERROR  : 0x00005f03b2c9c59f <unknown>+0x5f03b2c9c59f (/data/local/tmp/ds2+0x9559f)
[10853][void ds2::Utils::PrintBacktrace()] ERROR  : 0x00005f03b2c84bfe <unknown>+0x5f03b2c84bfe (/data/local/tmp/ds2+0x7dbfe)
[10853][void ds2::Utils::PrintBacktrace()] ERROR  : 0x00005f03b2c847d2 <unknown>+0x5f03b2c847d2 (/data/local/tmp/ds2+0x7d7d2)
[10853][void ds2::Utils::PrintBacktrace()] ERROR  : 0x00005f03b2ca2dc8 <unknown>+0x5f03b2ca2dc8 (/data/local/tmp/ds2+0x9bdc8)
[10853][void ds2::Utils::PrintBacktrace()] ERROR  : 0x00005f03b2c646dd <unknown>+0x5f03b2c646dd (/data/local/tmp/ds2+0x5d6dd)
[10853][void ds2::Utils::PrintBacktrace()] ERROR  : 0x00005f03b2c60977 <unknown>+0x5f03b2c60977 (/data/local/tmp/ds2+0x59977)
[10853][void ds2::Utils::PrintBacktrace()] ERROR  : 0x00007be7296c09f0 __libc_init+0x60 (/apex/com.android.runtime/lib64/bionic/libc.so+0x529f0)
```
Addresses relative to the module can be decoded on the build machine
with `lldb-symbolizer`:
```
$ llvm-symbolizer --obj=./build/ds2 <stack.txt 
ds2::Utils::PrintBacktrace()
??:0:0

ds2::Log(int, char const*, char const*, char const*, ...)
??:0:0

ds2::Target::Linux::Process::wait()
??:0:0

ds2::GDBRemote::DebugSessionImplBase::onResume(ds2::GDBRemote::Session&, std::__ndk1::vector<ds2::GDBRemote::ThreadResumeAction, std::__ndk1::allocator<ds2::GDBRemote::ThreadResumeAction>> const&, ds2::GDBRemote::StopInfo&)
??:0:0

ds2::GDBRemote::Session::Handle_vCont(ds2::GDBRemote::ProtocolInterpreter::Handler const&, std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char>> const&)
??:0:0

ds2::GDBRemote::ProtocolInterpreter::onCommand(std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char>> const&, std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char>> const&)
??:0:0

ds2::GDBRemote::ProtocolInterpreter::onPacketData(std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char>> const&, bool)
??:0:0

ds2::GDBRemote::SessionBase::receive(bool)
??:0:0

RunDebugServer(ds2::Host::Channel*, ds2::GDBRemote::SessionDelegate*)
main.cpp:0:0

main
??:0:0
```
NOTE: file location debug information is also shown if ds2 is compiled
with `-g`.

Author: andrurogerz

Sources/Utils/Backtrace.cpp

@@ -11,7 +11,7 @@
 #include "DebugServer2/Utils/Backtrace.h"
 #include "DebugServer2/Utils/Log.h"
 
-#if defined(OS_DARWIN) || defined(__GLIBC__)
+#if defined(ANDROID) || defined(OS_DARWIN) || defined(__GLIBC__)
 #include <cxxabi.h>
 #include <dlfcn.h>
 #include <execinfo.h>
@@ -22,20 +22,59 @@
 #include <sstream>
 #endif
 
+#if defined(ANDROID)
+// Android provides libunwind in the NDK instead of glibc backtrace
+#include <unwind.h>
+#endif
+
 namespace ds2 {
 namespace Utils {
 
-#if defined(OS_DARWIN) || (defined(__GLIBC__) && !defined(PLATFORM_TIZEN))
+#if defined(ANDROID)
+struct unwind_state {
+    void **buffer_cursor;
+    void **buffer_end;
+};
+
+static _Unwind_Reason_Code UnwindTrace(struct _Unwind_Context* context,
+                                       void* arg) {
+  struct unwind_state* state = reinterpret_cast<struct unwind_state*>(arg);
+  uintptr_t ip = _Unwind_GetIP(context);
+  if (ip != 0) {
+    if (state->buffer_cursor >= state->buffer_end)
+      return _URC_END_OF_STACK; // Backtrace will be truncated
+
+    *(state->buffer_cursor++) = reinterpret_cast<void*>(ip);
+  }
+  return _URC_NO_REASON;
+}
+
+// An approximation of glibc's backtrace using _Unwind_Backtrace
+static int UnwindBacktrace(void *stack_buffer[], int stack_size) {
+  unwind_state state = {
+    .buffer_cursor = stack_buffer,
+    .buffer_end = stack_buffer + stack_size,
+  };
+
+  _Unwind_Backtrace(UnwindTrace, &state);
+  return state.buffer_cursor - stack_buffer;
+}
+#elif defined(OS_DARWIN) || (defined(__GLIBC__) && !defined(PLATFORM_TIZEN))
+// Forward to glibc's backtrace
+static int UnwindBacktrace(void *stack_buffer[], int stack_size) {
+  return ::backtrace(stack_buffer, stack_size);
+}
+#endif
+
+#if defined(ANDROID) || defined(OS_DARWIN) || (defined(__GLIBC__) && !defined(PLATFORM_TIZEN))
 static void PrintBacktraceEntrySimple(void *address) {
   DS2LOG(Error, "%" PRI_PTR, PRI_PTR_CAST(address));
 }
-#endif
 
-#if defined(OS_DARWIN) || (defined(__GLIBC__) && !defined(PLATFORM_TIZEN))
 void PrintBacktrace() {
   static const int kStackSize = 100;
   static void *stack[kStackSize];
-  int stackEntries = ::backtrace(stack, kStackSize);
+  int stackEntries = UnwindBacktrace(stack, kStackSize);
 
   for (int i = 0; i < stackEntries; ++i) {
     Dl_info info;
@@ -60,10 +99,12 @@ void PrintBacktrace() {
       name = "<unknown>";
     }
 
-    DS2LOG(Error, "%" PRI_PTR " %s+%#" PRIxPTR " (%s)", PRI_PTR_CAST(stack[i]),
+    DS2LOG(Error, "%" PRI_PTR " %s+%#" PRIxPTR " (%s+%#" PRIxPTR ")",
+           PRI_PTR_CAST(stack[i]),
            name,
            static_cast<char *>(stack[i]) - static_cast<char *>(info.dli_saddr),
-           info.dli_fname);
+           info.dli_fname,
+           static_cast<char *>(stack[i]) - static_cast<char *>(info.dli_fbase));
 
     ::free(demangled);
   }