AtomicPair operations are only available with some instructions
introduced in version R6. Add support for needed instructions.

Change-Id: I808d6ed5b5efafd638846ec599941ebc71d90e23
Reviewed-on: https://chromium-review.googlesource.com/c/1251526Reviewed-by: Ivica Bogosavljevic <ibogosavljevic@wavecomp.com>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Sreten Kovacevic <skovacevic@wavecomp.com>
Cr-Commit-Position: refs/heads/master@{#56379}