[PGO][Offload] Don't define GPU entrypoint on Darwin (#132966)

This PR partially reverts 83e180c and instead opts to not define the GPU entry
point on Darwin platforms. Marking `__llvm_write_custom_profile` as used
was causing issues on embedded platforms.

compiler-rt/lib/profile/InstrProfilingFile.c

@@ -1273,6 +1273,7 @@ COMPILER_RT_VISIBILITY int __llvm_profile_set_file_object(FILE *File,
   return 0;
 }
 
+#ifndef __APPLE__
 int __llvm_write_custom_profile(const char *Target,
                                 const __llvm_profile_data *DataBegin,
                                 const __llvm_profile_data *DataEnd,
@@ -1381,5 +1382,6 @@ int __llvm_write_custom_profile(const char *Target,
 
   return ReturnValue;
 }
+#endif
 
 #endif