[mips][mips64r6] Define _MIPS_FPSET, __mips_fpr, and __mips_nan2008 correctly on MIPS32r6/MIPS64r6

Summary:
This removes the need to pass -mnan=2008 explicitly to be able to compile
the test-suite for MIPS32r6/MIPS64r6.

Differential Revision: http://reviews.llvm.org/D4433