The Makefile version seems to contain a bug. Over the years, the
directory structure of the `rust-src` component changed as the source
tree directory structure changed. `libstd` is no longer a thing directly
under `root/lib/rustlib/src/rust/src/`, it is moved to
`root/lib/rustlib/src/rust/library/std`.
Co-authored-by: Oneirical <manchot@videotron.ca>