[SECP256K1] Make lambda constant accessible

Summary:
Partial backport 6/11 of secp256k1 PR830:
https://github.com/bitcoin-core/secp256k1/pull/830/commits/fe7fc1fda8675aa9d79dae54a1b8b3cd06abcf81

Depends on D8040.

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8042