Enable memory sanitizer for PPC64

Summary:
This patch adds basic memory sanitizer support for PPC64. PR23219.

I have further patches ready to enable it in LLVM and Clang, and to fix
most of the many failing tests in check-msan.

Reviewers: kcc, willschm, samsonov, wschmidt, eugenis

Reviewed By: eugenis

Subscribers: wschmidt, llvm-commits

Differential Revision: http://reviews.llvm.org/D10648

llvm-svn: 240623