[libc++] Remove unused generalization from the libc++ benchmarking pipeline (#227369)

I initially tried reusing the same workflow file to run libc++ and
libstdc++ benchmarks, but since then I have given up on that approach.
As part of that, I had generalized the workflow and machines.json to
support not building the library. Since that capability won't be useful,
I am removing the generalization to simplify (a bit) the workflow and
machines.json.

Also introduce a `pr-benchmarks` key in machines.json to explicitly opt
into the PR benchmarking job: otherwise we'd have to implicitly key off
of the existence of `cmake-cache`, which is awkward.

GitOrigin-RevId: 6ba7cc00c940912ee6f8663a9e3026994920d478