[PPC] Undefine __ppc64__ to match GCC

GCC only defines `__ppc64__` for darwin while the darwin support has been
removed from llvm-project. The existence of `__ppc64__` makes some software
think we are compiling for big-endian PowerPC Mac; also it lures users to write
code which is not portable to GCC.

It is straightforward if a distro wants to keep the macro: add
`-D__ppc64__=1` to a Clang configuration file.

Reviewed By: thesamesam, nemanjai

Differential Revision: https://reviews.llvm.org/D137511