ARM: use LLVM's atomicrmw instructions when ldrex/strex are available.

Having some kind of weird kernel-assisted ABI for these when the
native instructions are available appears to be (and should be) the
exception; OSs have been gradually opting in for years and the code
was getting silly.

So let LLVM decide whether it's possible/profitable to inline them by
default.

Patch by Phoebe Buckheister.