Provide a way to bench memory allocators

[Mimalloc-bench]( https://github.com/daanx/mimalloc-bench ) would like to [make use of fleetbench]( https://github.com/daanx/mimalloc-bench/issues/162), but unfortunately, it seems that the usage of tcmalloc is hardcoded into the binaries. Would it be possible to make it optional, since mimalloc-bench is using `LD_PRELOAD` to override `malloc/free/…`.