libc: fix strtold NaN representation on riscv

Regenerate gd_qnan.h on riscv using the qnan.c
config tool found in contrib/gdtoa.

This fixes the following tests in CI:
lib/libc/stdio/scanfloat_test:infinities_and_nans
lib/libc/stdlib/strtod_test:strtold_nan

Reviewed by: jrtc27
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D57405