LibC: Use __builtin_thread_pointer for _get_uthread()

This generates much nicer assembly as it does not have to read thread
pointer for every access to TCB (errno, cancel_state, cancelled) and
instead it can read it once and use the same value for all accesses

userspace/libraries/LibC/include/bits/types/uthread.h

@@ -48,19 +48,7 @@ struct uthread
 	uintptr_t dtv[1 + 256];
 };
 
-#if defined(__x86_64__)
-#define _get_uthread() ({ \
-		struct uthread* _uthread; \
-		__asm__ volatile("movq %%fs:0, %0" : "=r"(_uthread)); \
-		_uthread; \
-	})
-#elif defined(__i686__)
-#define _get_uthread() ({ \
-		struct uthread* _uthread; \
-		__asm__ volatile("movl %%gs:0, %0" : "=r"(_uthread)); \
-		_uthread; \
-	})
-#endif
+#define _get_uthread() ((struct uthread*)__builtin_thread_pointer())
 
 __END_DECLS