Build time/size benchmark: document/save which compiler/linker (+version) is used

[rburing]

This is relevant info for a reproducible benchmark, and for comparing benchmarks.

[Calinou]

Like #113, this information can be added to new benchmarks (although the approach is different, as you'd have to do it using run-benchmarks.sh). However, I don't remember the exact compiler/linker version used for old benchmarks, so we could only estimate the GCC/ld version based on what was present in the Fedora 38/40/42 repositories.

All I can say is that I've always used the system defaults, that is, GCC + GNU ld.