[mips] Added -mfp64 and -mfp32 options.

These options specify 64-bit FP registers and 32-bit FP registers respectively.

When using -mfp32, the FPU has 16x double-precision registers overlapping with
the 32x single-precision registers (each double-precision register overlaps
two single-precision registers).

When using -mfp64, the FPU has 32x double-precision registers overlapping with
the 32x single-precision registers (each double-precision register overlaps
with one single-precision register and has an additional 32-bits).

MSA requires -mfp64.