MIPS: fix build from IR files, nan2008 and FpAbi

When we use llc or lld to compiler IR files, the features +nan2008 and +fpxx/+fp64 are not used.
Thus wrong format files are produced.

In IR files, the attributes are only set for function while not the whole compile units.
So we extract the attributes from the first function and use it for the whole unit.

isFPXXDefault: for o32, the FPXX should always be the default, no matter about the vendors.
Of course some distributions with FP64 default enabled should be listed explicit.
Let's add them in future if we know about one.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D140270