[mips] Pass nan2008 info to the back-end.

Summary: The initial support for NaN2008 was added to the back-end in r206396.

Reviewers: atanasyan

Reviewed By: atanasyan

Differential Revision: http://reviews.llvm.org/D3448

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208220 91177308-0d34-0410-b5e6-96231b3b80d8