Apple: Add pthread implementation of Thread class

This allows Apple platforms to override the default stack size of
a thread in the WorkerThreadPool, which is 512KiB by default.

This must be increased, as SPIRV-Cross, used by the Metal driver, can
use deeply nested stacks, as can debug builds.

drivers/unix/thread_posix.cpp

@@ -35,6 +35,11 @@
 #include "core/os/thread.h"
 #include "core/string/ustring.h"
 
+#if defined(PLATFORM_THREAD_OVERRIDE) && defined(__APPLE__)
+void init_thread_posix() {
+}
+#else
+
 #ifdef PTHREAD_BSD_SET_NAME
 #include <pthread_np.h>
 #endif
@@ -73,4 +78,6 @@ void init_thread_posix() {
 	Thread::_set_platform_functions({ .set_name = set_name });
 }
 
+#endif // PLATFORM_THREAD_OVERRIDE && __APPLE__
+
 #endif // UNIX_ENABLED