Improve 64bit atomics support.

When adding atomics back in b64d92f1, I added 64bit support as
optional; there wasn't yet a direct user in sight.  That turned out to
be a bit short-sighted, it'd already have been useful a number of times.

Add a fallback implementation of 64bit atomics, just like the one we
have for 32bit atomics.

Additionally optimize reads/writes to 64bit on a number of platforms
where aligned writes of that size are atomic. This can now be tested
with PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY.

Author: Andres Freund
Reviewed-By: Amit Kapila
Discussion: https://postgr.es/m/20160330230914.GH13305@awork2.anarazel.de