The implementation is based on the work by Bernstein and Yang
(https://eprint.iacr.org/2019/266)
"Fast constant-time gcd computation and modular inversion".
It fixes the old mp_gcd and s_mp_invmod_odd_m functions. The patch also fixes
mpl_significant_bits s_mp_div_2d and s_mp_mul_2d by having less control flow to
reduce side-channel leaks.
Co-authored by : Billy Bob Brumley