We weren't particularly maintaining barrier.h before 9.4, because nothing
was using it in those branches.  Well, nothing until commit 37de8de9 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 e5592c61, 89779bf2,
and 747ca669, 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.