Avoid adding fastcall attribute on linux aarch64

fastcall should only be added for 32 bit x86, as explained in the
comment. On some linux aarch64 compilers, none of the existing defines
are set, and __aarch64__ is set instead. Adding this avoids warnings
like:

```
In file included from /home/runner/haxelib/hxcpp/git/include/hxcpp.h:336:
/home/runner/haxelib/hxcpp/git/include/hx/Object.h:60:65: warning: ‘fastcall’ attribute directive ignored [-Wattributes]
   60 | typedef void (CPPIA_CALL *StackExecute)(struct StackContext *ctx);
      |                                                                 ^
In file included from /home/runner/haxelib/hxcpp/git/include/hxcpp.h:336:
/home/runner/haxelib/hxcpp/git/include/hx/Object.h:60:65: warning: ‘fastcall’ attribute directive ignored [-Wattributes]
   60 | typedef void (CPPIA_CALL *StackExecute)(struct StackContext *ctx);
      |                                                                 ^
```

Author: tobil4sk

include/hx/Object.h

@@ -43,7 +43,7 @@ HXCPP_EXTERN_CLASS_ATTRIBUTES null BadCast();
 // CPPIA_CALL = fastcall on x86(32), nothing otherwise
 #if (defined(_WIN32) && !defined(_M_X64) && !defined(__x86_64__) && !defined(_ARM_) ) || \
    defined(HXCPP_X86) || defined(__i386__) || defined(__i386) || \
-   (!defined(_WIN32) && !defined(_ARM_) && !defined(__arm__) && !defined(__x86_64__) )
+   (!defined(_WIN32) && !defined(_ARM_) && !defined(__arm__) && !defined(__aarch64__) && !defined(__x86_64__) )
 
    #if defined(__GNUC__) && !defined(__APPLE__) && !defined(EMSCRIPTEN)
       #define CPPIA_CALL __attribute__ ((fastcall))