We weren't particularly maintaining barrier.h before 9.4, because nothing
was using it in those branches. Well, nothing until commit 37de8de9e got
back-patched. That broke 9.2 and 9.3 for some non-mainstream platforms
that we haven't been testing in the buildfarm, including icc on ia64,
HPPA, and Alpha.
This commit effectively back-patches commits e5592c61a, 89779bf2c,
and 747ca6697, though I did it just by copying the file (less copyright
date updates) rather than by cherry-picking those commits.
Per an attempt to run gaur and pademelon over old branches they've
not been run on since ~2013.
f4f4f6990e