The code of kArmWord32AtomicPairStore is not self-explanatory,
especially why {ldrexd} is used. This CL adds some documentation to
make it more understandable.

Additionally this CL changes the code generation to use {cmp} instead
of {teq}. It's the preferred (idiomatic) sequence on Arm.

R=v8-arm-ports@googlegroups.com

Bug: v8:10155
Change-Id: I32946a333c352250045dbd8872862529a63c8772
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2129638Reviewed-by: Jacob Bramley <jacob.bramley@arm.com>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66987}