pgsql: Fix spinlock implementation for some !solaris sparc platforms.

Fix spinlock implementation for some !solaris sparc platforms.

Some Sparc CPUs can be run in various coherence models, ranging from
RMO (relaxed) over PSO (partial) to TSO (total). Solaris has always
run CPUs in TSO mode while in userland, but linux didn't use to and
the various *BSDs still don't. Unfortunately the sparc TAS/S_UNLOCK
were only correct under TSO. Fix that by adding the necessary memory
barrier instructions. On sparcv8+, which should be all relevant CPUs,
these are treated as NOPs if the current consistency model doesn't
require the barriers.

Discussion: 20140630222854.GW26930@awork2.anarazel.de

Will be backpatched to all released branches once a few buildfarm
cycles haven't shown up problems. As I've no access to sparc, this is
blindly written.

Branch
------
REL9_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/d0b7ffc0f6d4e1d0e0d640ec4cc60ecd81e9ca2b

Modified Files
--------------
src/backend/port/tas/sunstudio_sparc.s |    2 ++
src/include/storage/s_lock.h           |   49 +++++++++++++++++++++++++++++++-
2 files changed, 50 insertions(+), 1 deletion(-)