pgsql: For all ppc compilers,implement pg_atomic_fetch_add_ with inlin

For all ppc compilers, implement pg_atomic_fetch_add_ with inline asm.

This is more like how we handle s_lock.h and arch-x86.h.  This does not
materially affect code generation for gcc 7.2.0 or xlc 13.1.3.

Reviewed by Tom Lane.

Discussion: https://postgr.es/m/20190831071157.GA3251746@rfd.leadboat.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e7ff59686eacf5021fb84be921116986c3828d8a

Modified Files
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configure                              | 40 ++++++++++++++
configure.in                           | 20 +++++++
src/include/pg_config.h.in             |  3 ++
src/include/port/atomics/arch-ppc.h    | 98 ++++++++++++++++++++++++++++++++++
src/include/port/atomics/generic-xlc.h | 66 -----------------------
5 files changed, 161 insertions(+), 66 deletions(-)