"optimize" arm __pthread_self

actually this is just to avoid gcc being stupid and refusing to inline
the function version, even when the size cost is essentially identical
whether it's inlined or not.

arch/arm/pthread_arch.h

-static inline struct pthread *__pthread_self()
-{
-	return ((void *(*)(void))0xffff0fe0)();
-}
+#define __pthread_self ((pthread_t (*)(void))0xffff0fe0)
 
 #define CANCEL_REG_SP 16
 #define CANCEL_REG_IP 18