Reorg FloatParts to use QEMU_GENERIC.
Begin replacing the Berkeley float128 routines with FloatParts128.
- includes a new implementation of float128_muladd
- includes the snan silencing that was missing from
float{32,64}_to_float128 and float128_to_float{32,64}.
- does not include float128_min/max* (written but not yet reviewed).