Clang: Define macro _MIPS_SPFPSET

GCC defines this macro for how many single-precision floating point registers
can be used.

If the -mno-odd-spreg option is given, it will be 16; if either -mno-odd-spreg
nor -modd-spreg are given, we set it to 16 for FPXX.

Reviewed By: theraven

Differential Revision: https://reviews.llvm.org/D157896