[mips] Don't propagate -mfpxx by default if soft/single float were also set.

Summary:
If the driver is only given -msoft-float/-mfloat-abi=soft or -msingle-float,
we should refrain from propagating -mfpxx, unless it was explicitly given on the
command line.

Reviewers: atanasyan, dsanders

Reviewed By: atanasyan, dsanders

Subscribers: cfe-commits, mpf

Differential Revision: http://reviews.llvm.org/D10387

llvm-svn: 239818