arm_acle: Implement swap intrinsic

Insert the LDREX/STREX instruction sequence specified in ARM ACLE 2.0,
as SWP instruction is deprecated since ARMv6.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216446 91177308-0d34-0410-b5e6-96231b3b80d8