Rollup merge of #147419 - cuviper:bootstrap-rustc-libs, r=Zalathar,jieyouxu

bootstrap: add `Builder::rustc_cmd` that includes the lib path

When building with `rust.rpath = false`, every `rustc` invocation needs
to include the library path as well. I particularly ran into this in
`generate_target_spec_json_schema` when testing 1.91-beta in Fedora,
where we do disable rpath for our system builds. The new helper function
will hopefully encourage the right thing going forward.

Author: Zalathar