fix printf precision specifier for hex floats on non-ld80 archs

fix printf precision specifier for hex floats on non-ld80 archs

the code to perform rounding to the desired precision wrongly assumed
the long double mantissa was an integral number of nibbles (hex
digits) in length. this is true for 80-bit extended precision (64-bit
mantissa) but not for double (53) or quad (113).

scale the rounding value by 1<<(LDBL_MANT_DIG%4) to compensate.