Comment: scan_num also handles hexadecimal floats now.
Modify the hexfp exponent explicitly in each long double case.
double-double needs the first mantissa bits of doubles.
Reduce double-double %a to single-double for now.

How to exactly handle the second double is unknown, so for
now extract the mantissa bits only from the first double.
(Known platforms aix-ppc and linux-ppc.)
Quadruple float BE test triggering had a typo.
Add todo comment about special hexfp values.