pgsql: Use __sync_lock_test_and_set() for spinlocks on ARM, if availabl

Use __sync_lock_test_and_set() for spinlocks on ARM, if available.

Historically we've used the SWPB instruction for TAS() on ARM, but this
is deprecated and not available on ARMv6 and later. Instead, make use
of a GCC builtin if available. We'll still fall back to SWPB if not,
so as not to break existing ports using older GCC versions.

Eventually we might want to try using __sync_lock_test_and_set() on some
other architectures too, but for now that seems to present only risk and
not reward.

Back-patch to all supported versions, since people might want to use any
of them on more recent ARM chips.

Martin Pitt

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/a2beb5b3d1d5848f85d78d8365be70bdb0337603

Modified Files
--------------
configure | 61 ++++++++++++++++++++++++++++++++++++++++++
configure.in | 11 +++++++
src/include/pg_config.h.in | 3 ++
src/include/storage/s_lock.h | 25 ++++++++++++++++-
4 files changed, 98 insertions(+), 2 deletions(-)