audit memcmp usage

[toidiu]

Resolved issues:

Description of changes:

s2n codes should use s2n_constant_time_equals instead of memcmp.

This PR adds a check for uses of memcmp to codebuild/bin/grep_simple_mistakes.sh, which is run as part of CI for each PR. It notes the current uses of 'memcmp' and will flag any future PRs which add or remove uses.

This should give better visibility into new uses of 'memcmp' while reviewing the PR and also help target/replace the current uses.

• failure case: https://github.com/aws/s2n-tls/pull/3059/checks?check_run_id=3655579669
• success case: commit - https://github.com/aws/s2n-tls/runs/3655595096

Issue to audit and replace memcmp usage: #3062

Call-outs:

Testing:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[camshaft]

Is there a tracking issue to actually fix the ones that are currently there? If so can you link it here? Otherwise please create one.