target-mips: use softfloat constants when possible

target-mips: use softfloat constants when possible

softfloat already has a few constants defined, use them instead of
redefining them in target-mips.

Rename FLOAT_SNAN32 and FLOAT_SNAN64 to FP_TO_INT32_OVERFLOW and
FP_TO_INT64_OVERFLOW as even if they have the same value, they are
technically different (and defined differently in the MIPS ISA).

Remove the unused constants.

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