Given a gcc-compatible xlc compiler, prefer xlc-style atomics.
Given a gcc-compatible xlc compiler, prefer xlc-style atomics.

This evades a ppc64le "IBM XL C/C++ for Linux" compiler bug.  Back-patch
to 9.5, where the atomics facility was introduced.

zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl