make aarch64 atomic_arch.h report that it defines pointer-sized ll/sc

at present this is done only for consistency, since this file defines
its own a_cas_p rather than using the new generic one from atomic.h
added in commit 225f6a6b5b7173b6655e4f5d49b5b9fea70bf3bb. these
definitions may however be useful if we ever need to add other
pointer-sized atomic operations.
1 file changed