Atomic load/store on ARM/Thumb.

I don't really like the patterns, but I'm having trouble coming up with a
better way to handle them.

I plan on making other targets use the same legalization
ARM-without-memory-barriers is using... it's not especially efficient, but
if anyone cares, it's not that hard to fix for a given target if there's
some better lowering.

llvm-svn: 138621