Fix false warnings with readonly atomics

Code like:

    #include <stdatomic.h>
    _Atomic const int ai; int aiGet(void){ return atomic_load(&ai); }

has been raising "warning: assignment of read-only location".

This is due to `__typeof__ (*ptr) tmp;` not rvalue-converting the target
type.

include/stdatomic.h

@@ -100,7 +100,7 @@ typedef struct {
        __val; })
 #define atomic_load_explicit(object, order)                               \
     ({ __typeof__ (object) ptr = (object);                                \
-       __typeof__ (*ptr) tmp;                                             \
+       __typeof__ ((void)0,*ptr) tmp;                                             \
        __atomic_load (ptr, &tmp, (order));                                \
        tmp;                                                               \
     })