[PowerPC][msan] Update msan to handle changed memory layouts in newer kernels

In more recent Linux kernels (including those with 47 bit VMAs) the layout of
virtual memory for powerpc64 changed causing the memory sanitizer to not
work properly. This patch adjusts a bit mask in the memory sanitizer to work
on the newer kernels while continuing to work on the older ones as well.

This is the non-runtime part of the patch and finishes it. ref: r317802

Tested on several 4.x and 3.x kernel releases.

llvm-svn: 318045