[SECP256K1] Don't use reserved identifiers memczero and benchmark_verify_t
Summary:
As identified in #829 and #833. Fixes #829. Since we touch this anyway, this commit additionally makes the identifiers in the benchmark files a little bit more consistent. Typedef (u)int128_t only when they're not provided by the compiler
Backport of secp256k1#835
Test Plan:
ninja check-secp256k1 bench-secp256k1
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D9373