use __attribute__((const)) on arm __pthread_self function

arch/arm/pthread_arch.h

@@ -1,4 +1,6 @@
-#define __pthread_self ((pthread_t (*)(void))0xffff0fe0)
+typedef pthread_t (*__pthread_self_func_t)(void) __attribute__((const));
+
+#define __pthread_self ((__pthread_self_func_t)0xffff0fe0)
 
 #define CANCEL_REG_SP 16
 #define CANCEL_REG_IP 18