pgsql: Further atomic ops portability improvements and bug fixes.

Further atomic ops portability improvements and bug fixes.

* Don't play tricks for a more efficient pg_atomic_clear_flag() in the
generic gcc implementation. The old version was broken on gcc < 4.7
on !x86 platforms. Per buildfarm member chipmunk.
* Make usage of __atomic() fences depend on HAVE_GCC__ATOMIC_INT32_CAS
instead of HAVE_GCC__ATOMIC_INT64_CAS - there's platforms with 32bit
support that don't support 64bit atomics.
* Blindly fix two superflous #endif in generic-xlc.h
* Check for --disable-atomics in platforms but x86.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/f9f07411a5b879b232ade66fece7071bd2eb5c26

Modified Files
--------------
src/include/port/atomics/arch-x86.h | 13 +++++++++++++
src/include/port/atomics/generic-acc.h | 11 ++++++++++-
src/include/port/atomics/generic-gcc.h | 14 ++++----------
src/include/port/atomics/generic-msvc.h | 8 ++++++++
src/include/port/atomics/generic-sunpro.h | 8 ++++++++
src/include/port/atomics/generic-xlc.h | 10 ++++++++--
6 files changed, 51 insertions(+), 13 deletions(-)