[DFSAN][MIPS][LLVM] Defining ShadowPtrMask variable for MIPS64

Patch by Kumar Sukhani!

corresponding compiler-rt patch: http://reviews.llvm.org/D6437
clang patch: http://reviews.llvm.org/D6147

Differential Revision: http://reviews.llvm.org/D6459

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223516 91177308-0d34-0410-b5e6-96231b3b80d8