[PowerPC] Remove unnecessary preprocessor checking.

The tests !defined(__ppc__) && !defined(__powerpc__) are not needed
or helpful when verifying that code is being compiled for a 64-bit
target.  The simpler test provided by this revision is sufficient to
tell if the target is 64-bit.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187318 91177308-0d34-0410-b5e6-96231b3b80d8