Auto merge of #38008 - bluss:rustbuild-benches, r=alexcrichton

Add rustbuild command `bench`

Add command bench to rustbuild, so that `./x.py bench <path>` can compile and run benchmarks.

`./x.py bench --stage 1 src/libcollections` and `./x.py bench --stage 1 src/libstd` should both compile well. Just `./x.py bench` runs all benchmarks for the libstd crates.

Fixes #37897

diff --cc src/bootstrap/lib.rs
Simple merge