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.

llvm-svn: 178559