pgsql: For ppc gcc, implement 64-bit compare_exchange and fetch_add wit

For ppc gcc, implement 64-bit compare_exchange and fetch_add with asm.

While xlc defines __64BIT__, gcc does not.  Due to this oversight in
commit 30ee5d17c20dbb282a9952b3048d6ad52d56c371, gcc builds continued
implementing 64-bit atomics by way of intrinsics.  Back-patch to v13,
where that commit first appeared.

Reviewed by Tom Lane.

Discussion: https://postgr.es/m/20201011051043.GA1724101@rfd.leadboat.com

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d41cb63ff4d114d856837fbf61ba2872c5076ac2

Modified Files
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src/include/port/atomics/arch-ppc.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)