Minimally support compiling Odin on FreeBSD arm64

This is enough to get Odin itself compiling and the demo running.

src/threading.cpp

@@ -492,6 +492,8 @@ gb_internal u32 thread_current_id(void) {
 	thread_id = gettid();
 #elif defined(GB_SYSTEM_HAIKU)
 	thread_id = find_thread(NULL);
+#elif defined(GB_SYSTEM_FREEBSD)
+	thread_id = pthread_getthreadid_np();
 #else
 	#error Unsupported architecture for thread_current_id()
 #endif