[Cortex-M0] Atomic lowering

Summary: ARMv6m supports dmb etc fench instructions but not ldrex/strex etc. So for some atomic load/store, LLVM should inline instructions instead of lowering to __sync_ calls.

Reviewers: rengolin, efriedma, t.p.northover, jmolloy

Subscribers: efriedma, aemerson, llvm-commits

Differential Revision: https://reviews.llvm.org/D26120

llvm-svn: 285969