Fix PR15630:  Replace faulty stdcx. with stwcx.

When doing a partword atomic operation, a lwarx was being paired with
a stdcx. instead of a stwcx. when compiling for a 64-bit target.  The
target has nothing to do with it in this case; we always need a stwcx.

Thanks to Kai Nacke for reporting the problem.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178559 91177308-0d34-0410-b5e6-96231b3b80d8