[tsan] Test __tsan_test_only_on_fork only on Mac (llvm#96597)

According to https://reviews.llvm.org/D114250
this was to handle Mac specific issue, however
the test is Linux only.

The test effectively prevents to lock main allocator
on fork, but we do that on Linux for other
sanitizers for years, and need to do the same
for TSAN to avoid deadlocks.

Author: vitalybuka

compiler-rt/lib/tsan/rtl/tsan_rtl.cpp

@@ -35,8 +35,10 @@ extern "C" void __tsan_resume() {
   __tsan_resumed = 1;
 }
 
+#if SANITIZER_APPLE
 SANITIZER_WEAK_DEFAULT_IMPL
 void __tsan_test_only_on_fork() {}
+#endif
 
 namespace __tsan {
 
@@ -828,7 +830,9 @@ void ForkBefore(ThreadState* thr, uptr pc) SANITIZER_NO_THREAD_SAFETY_ANALYSIS {
   // Disables memory write in OnUserAlloc/Free.
   thr->ignore_reads_and_writes++;
 
+#  if SANITIZER_APPLE
   __tsan_test_only_on_fork();
+#  endif
 }
 
 static void ForkAfter(ThreadState* thr) SANITIZER_NO_THREAD_SAFETY_ANALYSIS {