[secp256k1] refactor: Make generator a parameter of ecmult_gen_create_prec_table
Summary:
This is a partial backport of secp256k1#988
https://github.com/bitcoin-core/secp256k1/pull/988/commits/e1a76530db40b8aa8953717d9f984b6bdf845308
Depends on D19386
Test Plan: ninja check-secp256k1
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D19387