Preventing compiler optimizations in benchmarks without a memory fence

Summary:

• Added accumulators and checks on benchmarks so they won't get optimized out
• Modified bench_scalar_split so it won't get optimized out

This is a backport of libsecp256k1 PR678

Test Plan:

ninja check-secp256k1
ninja bench-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5374