target-mips: use the softfloat floatXX_muladd functions

target-mips: use the softfloat floatXX_muladd functions

Use the new softfloat floatXX_muladd() functions to implement the madd,
msub, nmadd and nmsub instructions. At the same time replace the name of
the helpers by the name of the instruction, as the only reason for the
previous names was to keep the macros simple.

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