[Sanitizers] Do not call internal_sigdelset() on non-Linux

[samsonov]

LGTM. If internal_sigdelset is available on FreeBSD (we just shouldn't call it), then I'd prefer if(SANITIZER_LINUX) instead of #if. Anyway, feel free to submit.

lib/sanitizer_common/sanitizer_linux.cc

bool IsDeadlySignal(int signum) {
   return (signum == SIGSEGV || signum == SIGBUS) && common_flags()->handle_segv;
 }
 
 #ifndef SANITIZER_GO
 void *internal_start_thread(void(*func)(void *arg), void *arg) {
   // Start the thread with signals blocked, otherwise it can steal user signals.
   __sanitizer_sigset_t set, old;
   internal_sigfillset(&set);
+#if SANITIZER_LINUX
   // Glibc uses SIGSETXID signal during setuid call. If this signal is blocked
   // on any thread, setuid call hangs (see test/tsan/setuid.c).
   internal_sigdelset(&set, 33);
+#endif
   internal_sigprocmask(SIG_SETMASK, &set, &old);
   void *th;
   real_pthread_create(&th, 0, (void*(*)(void *arg))func, arg);
   internal_sigprocmask(SIG_SETMASK, &old, 0);
   return th;
 }
 
 void internal_join_thread(void *th) {