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