remove ancient clang workaround from powerpc pthread_arch.h asm

versions of clang all the way back to 3.1 lack the bug this was
purportedly working around.

arch/powerpc/pthread_arch.h

 static inline struct pthread *__pthread_self()
 {
-#ifdef __clang__
-	char *tp;
-	__asm__ __volatile__ ("mr %0, 2" : "=r"(tp) : : );
-#else
 	register char *tp __asm__("r2");
 	__asm__ __volatile__ ("" : "=r" (tp) );
-#endif
 	return (pthread_t)(tp - 0x7000 - sizeof(struct pthread));
 }