Implemented ftentox and fixed fscale again

- (Badly) worked around a clang bug to implement ftentox
- (Very badly) fixed another edge case in fscale
- Found another bug in softfloat (floatx80_to_int32)
-- Not so sure anymore that my floatx80/subnormal math fix works right.
   Just figured out that x80's exp is relative to the second mantissa
   bit. float128{exp=0x3fff, man=0} == floatx80{exp=0x3fff, man=0x800...}
   even though x80 has an explicit bit, the exponents are the same...