target-mips: Unfuse {,N}M{ADD,SUB}.fmt

Turn MADD.fmt, MSUB.fmt, NMADD.fmt and NMSUB.fmt from fused to unfused
operations, so that they behave in the same way as a separate multiplication
and addition.  The instructions were only fused in early MIPS IV processors.

Signed-off-by: Richard Sandiford <rdsandiford@googlemail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>