Add definitions for store barrier.

I usually call this a "store/store barrier" for maximum clarity, but
the common way of describing it is "store barrier" or "store fence".

This doesn't use "dmb st" yet since we're waiting on the toolchain
update, but it gets the various macros and inline functions in place
so we can use them in the VM.

Bug 3003477

git cherry-pick 2ba5eec3972b4ce46feb677116534fcd3d136e0a

Change-Id: Ifd2d3588be96aa529d490789436cf48c962021ba