ARM: implement correct atomic operations on v7M

ARM v7M has ldrex/strex but not ldrexd/strexd. This means 32-bit
operations should work as normal, but 64-bit ones are almost certainly
doomed.

Patch by Phoebe Buckheister.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211042 91177308-0d34-0410-b5e6-96231b3b80d8