pgsql: Improve 64bit atomics support.

Improve 64bit atomics support.

When adding atomics back in b64d92f1a, 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

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/e8fdbd58fe564a29977f4331cd26f9697d76fc40

Modified Files
--------------
src/backend/port/atomics.c | 65 +++++++++++++++++++++++++++++++++++-
src/include/port/atomics.h | 13 +++-----
src/include/port/atomics/arch-ia64.h | 3 ++
src/include/port/atomics/arch-ppc.h | 3 ++
src/include/port/atomics/arch-x86.h | 10 ++++++
src/include/port/atomics/fallback.h | 33 ++++++++++++++++++
src/include/port/atomics/generic.h | 22 +++++++++---
src/test/regress/regress.c | 4 ---
8 files changed, 136 insertions(+), 17 deletions(-)