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.

core/os/thread.h

@@ -119,6 +119,8 @@ private:
 public:
 	static void _set_platform_functions(const PlatformFunctions &p_functions);
 
+	_FORCE_INLINE_ static void yield() { std::this_thread::yield(); }
+
 	_FORCE_INLINE_ ID get_id() const { return id; }
 	// get the ID of the caller thread
 	_FORCE_INLINE_ static ID get_caller_id() {