[mips] Relax -mnan=2008 acceptance to permit MIPS32R2 and MIPS64R2.

Summary:
Strictly speaking, the MIPS*R2 ISA's should not permit -mnan=2008 since this
feature was added in MIPS*R3. However, other toolchains permit this and we
should do the same.

Reviewers: atanasyan

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D13057

llvm-svn: 248481