pgsql: Improve spinlock performance for HP-UX, ia64, non-gcc.

Improve spinlock performance for HP-UX, ia64, non-gcc.

At least on this architecture, it's very important to spin on a
non-atomic instruction and only retry the atomic once it appears
that it will succeed.  To fix this, split TAS() into two macros:
TAS(), for trying to grab the lock the first time, and TAS_SPIN(),
for spinning until we get it.  TAS_SPIN() defaults to same as TAS(),
but we can override it when we know there's a better way.

It's likely that some of the other cases in s_lock.h require
similar treatment, but this is the only one we've got conclusive
evidence for at present.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/c01c25fbe525869fa81237954727e1eb4b7d4a14

Modified Files
--------------
src/backend/storage/lmgr/s_lock.c |    2 +-
src/include/storage/s_lock.h      |   35 ++++++++++++++++++++++++-----------
2 files changed, 25 insertions(+), 12 deletions(-)