Wrap linux specific code for `internal_thread_proc`

src/threading.cpp

@@ -1,4 +1,4 @@
-#if defined(GB_SYSTEM_UNIX)
+#if defined(GB_SYSTEM_LINUX)
 #include <signal.h>
 #endif
 
@@ -361,10 +361,12 @@ void gb__thread_run(Thread *t) {
 	}
 #else
 	void *internal_thread_proc(void *arg) {
-		// NOTE: Don't permit any signal delivery to threads.
+	#if (GB_SYSTEM_LINUX)
+		// NOTE: Don't permit any signal delivery to threads on Linux.
 		sigset_t mask = {};
 		sigfillset(&mask);
 		GB_ASSERT_MSG(pthread_sigmask(SIG_BLOCK, &mask, nullptr) == 0, "failed to block signals");
+	#endif
 		
 		Thread *t = cast(Thread *)arg;
 		gb__thread_run(t);