Allow CXX bridge and Rust symbols in symbol export check

When building with SPIRV_ENABLE_RUST_TARGET_ENV, the shared library
includes Rust FFI symbols from the cxx crate. Update the symbol export
check to allow:
- cxxbridge1$ : CXX bridge runtime symbols
- spvtools$ffi$ : SPIRV-Tools Rust FFI symbols
- _R : Rust v0 mangled symbols

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: LegNeato

utils/check_symbol_exports.py

@@ -61,7 +61,10 @@ def check_library(library):
     #   _ZSt              :  something in the standard namespace
     #   _ZZN              :  something in a local scope and namespace
     #   _Z[0-9]+spv[A-Z_] :  C++ symbol starting with spv[A-Z_]
-    symbol_ok_pattern = re.compile(r'^(spv[A-Z]|_ZN|_ZSt|_ZZN|_Z[0-9]+spv[A-Z_])')
+    #   cxxbridge1\$      :  CXX bridge runtime symbols (Rust FFI)
+    #   spvtools\$ffi\$   :  SPIRV-Tools Rust FFI symbols
+    #   _R                :  Rust mangled symbols (v0 mangling)
+    symbol_ok_pattern = re.compile(r'^(spv[A-Z]|_ZN|_ZSt|_ZZN|_Z[0-9]+spv[A-Z_]|cxxbridge1\$|spvtools\$ffi\$|_R)')
 
     # In addition, the following pattern allowlists global functions that are added
     # by the protobuf compiler: