[CMake][PowerPC] Recognize LLVM_NATIVE_TARGET="ppc64le" as PowerPC

Summary:
This value is derived from the host triple, which on the machine
I'm currently using is `ppc64le-linux-redhat`. This change makes
LLVM compile.

Reviewers: nemanjai
Differential Revision: https://reviews.llvm.org/D57118

llvm-svn: 359242