ARM: implement ldrex, strex and clrex intrinsics

Intrinsics already existed for the 64-bit variants, so these support operations
of size at most 32-bits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186392 91177308-0d34-0410-b5e6-96231b3b80d8