Sync 9.2 and 9.3 versions of barrier.h with 9.4's version.

Sync 9.2 and 9.3 versions of barrier.h with 9.4's version.

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.