MIPS: math-emu: Reduce microMIPS bloat.

Move microMIPS32_to_MIPS32() to a separate file which only gets built
for mipsMIPS configurations; for other configurations the optimizer
eleminates calls to microMIPS32_to_MIPS32().

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>