This CL implements AtomicPair operators: Load, Store,
Add, Sub, Or, Xor, And, Exchange and CompareExchange using
runtime on MIPS32R2 and older. MIPS32R6 includes instructions
for 64-bit atomic access so they are implemented using those.

Change-Id: I1309c1ea4771480516ec5a92f7592533bdcb205c
Reviewed-on: https://chromium-review.googlesource.com/c/1326466Reviewed-by: Sreten Kovacevic <skovacevic@wavecomp.com>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ivica Bogosavljevic <ibogosavljevic@wavecomp.com>
Cr-Commit-Position: refs/heads/master@{#57496}