[clang][mips] Set __mips_fpr correctly for -mfpxx

Set __mips_fpr to 0 if o32 ABI is used with either -mfpxx
or none of -mfp32, -mfpxx, -mfp64 being specified.

Introduce additional checks:
-mfpxx is only to be used in conjunction with the o32 ABI.
report an error when incompatible options are provided.

Formerly no errors were raised when combining n32/n64 ABIs
with -mfp32 and -mfpxx.

There are other cases when __mips_fpr should be set to 0
that are not covered, ex. using o32 on a mips64 cpu
which is valid but not supported in the backend as of yet.

Differential Revision: https://reviews.llvm.org/D50557

llvm-svn: 340391