Values for P-521 have an odd number of limbs in 32-bit mode, which means we can't keep using `TOBN`, and also Montgomery-encoded values are different for 32-bit and 64-bit.
Values for P-521 have an odd number of limbs in 32-bit mode, which means we can't keep using `TOBN`, and also Montgomery-encoded values are different for 32-bit and 64-bit.