Makefile: Allow user to override bench targets

See merge request https://gitlab.com/gitlab-org/gitaly/-/merge_requests/5998



Merged-by: Toon Claes <toon@gitlab.com>
Approved-by: karthik nayak <knayak@gitlab.com>
Approved-by: Toon Claes <toon@gitlab.com>
Co-authored-by: Will Chandler <wchandler@gitlab.com>