parameterize ecmult_const over input size

Summary: This is a backport of secp56k1's PR522

Test Plan:

ninja check-secp256k1 bench-secp256k1

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D4900