[SECP256K1] Add secp256k1_ctz{32,64}_var functions

Summary:

These functions count the number of trailing zeroes in non-zero
integers.

Partial backport of secp256k1#831:
https://github.com/bitcoin-core/secp256k1/pull/831/commits/de0a643c3dc2c40a447e670cfa1c1683c79c9297

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9401