Teach Clang about the PPC64 memory sanitizer implementation.

Summary:
This is the Clang part of the PPC64 memory sanitizer implementation in
D10648.

Reviewers: kcc, eugenis, willschm, wschmidt, samsonov

Reviewed By: samsonov

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D10650

llvm-svn: 240628