[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
[DFSAN][MIPS] adding support of DFSAN for MIPS64

Minor changes to enable DFSAN on MIPS64

Patch by Kumar Sukhani!

Differential Revision: http://reviews.llvm.org/D6437