tcg/mips: use MUL instead of MULT on MIPS32 and above

tcg/mips: use MUL instead of MULT on MIPS32 and above

MIPS32 and later instruction sets have a multiplication instruction
directly operating on GPRs. It only produces a 32-bit result but
it is exactly what is needed by QEMU.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>