Add ARM64 (aarch64) support to s_lock.h.

Use the same gcc atomic functions as we do on newer ARM chips.
(Basically this is a copy and paste of the __arm__ code block,
but omitting the SWPB option since that definitely won't work.)

Back-patch to 9.2.  The patch would work further back, but we'd also
need to update config.guess/config.sub in older branches to make them
build out-of-the-box, and there hasn't been demand for it.

Mark Salter