[Mips] Generate warning for invalid '-mnan' and '-march' combinations

This patch generates a warning for invalid combination of '-mnan' and
'-march' options, it properly sets NaN encoding for a given '-march',
and it passes a proper NaN encoding to the assembler.

Patch by Vladimir Radosavljevic.

Differential Revision: http://reviews.llvm.org/D8170

llvm-svn: 234882