Fix printf(3) output of long doubles on RISC-V
When the RISC-V port was initially committed to FreeBSD, GCC would
generate 64-bit long doubles, and the definitions in _fpmath.h reflected
that. This was changed to 128-bit in GCC later that year , but the
definitions were never updated, despite the documented workaround. This
causes printf(3) and friends to interpret only the low 64-bits of a long
double in ldtoa, thereby printing incorrect values.
Update the definitions now that both clang and GCC generate 128-bit long