[mips][msa] Enable inlinse assembly for MSA.

Like GCC, this re-uses the 'f' constraint and a new 'w' print-modifier:
  asm ("ldi.w %w0, 1", "=f"(result));

Unlike GCC, the 'w' print-modifer is not _required_ to produce the intended
output. This is a consequence of differences in the internal handling of
the registers in each compiler. To be source-compatible between the
compilers, users must use the 'w' print-modifier.

MSA registers (including control registers) are supported in clobber lists.