[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

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@223517 91177308-0d34-0410-b5e6-96231b3b80d8