gallivm: fix issue with AtomicCmpXchg wrapper on llvm 3.5-3.8

These versions still need wrapper but already have both success and
failure ordering.
(Compile tested on llvm 3.3, 3.7, 3.8.)

v2: don't duplicate whole function (suggested by Brian).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111102

Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
1 file changed