[PATCH] [MSan] [MIPS] Adding support for MIPS64

[Kostya Serebryany]

> We are aware of the limitation of MSan 32bit however there are existing customers of MIPS32 which we think might be potential users of MSAN 32bit. 

Ok, let's try. 
We can not afford to have MIIPS-32 and not have x86-32 because for most developers there is 
no way to test on MIIPS-32. 
I would ask you to enable MSan on x86-32, make sure that all tests pass, 
that check-msan runs the 32-bit tests on x86-64 machines, 
that the bots (lab.llvm.org:8011/builders/sanitizer-x86_64-linux and lab.llvm.org:8011/builders/sanitizer-x86_64-linux-bootstrap/)
run these tests too.
Then we can enable msan on other 32-bit architecture(s).

http://reviews.llvm.org/D5906