[SystemZ] Add support for 128-bit atomic load/store/cmpxchg

This adds support for the main 128-bit atomic operations,
using the SystemZ instructions LPQ, STPQ, and CDSG.

Generating these instructions is a bit more complex than usual
since the i128 type is not legal for the back-end.  Therefore,
we have to hook the LowerOperationWrapper and ReplaceNodeResults
TargetLowering callbacks.

llvm-svn: 310094