[SECP256K1] Switch to our own memcmp function

Summary:

Fixes #823.

Backport 1/11 of secp256k1 PR830:
https://github.com/bitcoin-core/secp256k1/pull/830/commits/6173839c90553385171d560be8a17cbe167e3bef

Note: the first memcmp() in recovery/tests_impl.h was removed in
D2193.

Depends on D8036.

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8037