Preventing compiler optimizations in benchmarks without a memory fence
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

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