Make fallback implementation of pg_memory_barrier() work in 9.2 and 9.3.

Back-patch 9.4-era commit 44cd47c1 into 9.2 and 9.3.  As with
my back-patches of yesterday, this was not seen as necessary at the time
because we didn't expect barrier.h to need to work before 9.4, but
commit 37de8de9 invalidated that theory.

Per an attempt to run gaur and pademelon over old branches they've
not been run on since ~2013.