[msan] Teach sanitizers about the PPC64 ptrace syscall

Summary:
This fixes test/msan/Linux/syscalls.cc, and should also fix the ppc64
sanitizer buildbots which are currently failing in
"make check-sanitizer".

Reviewers: samsonov, wschmidt, eugenis

Reviewed By: eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10734

llvm-svn: 240692