mk: Add `-C metadata` for compiling crates we ship

mk: Add `-C metadata` for compiling crates we ship

This should re-enable all external builds of crates with the same name. Right
now Cargo doesn't pass `-C metadata` for the top-level library being compiled,
so if that library is called `libc`, for example, then it won't be able to link
to the standard library which *also* has a `libc` library compiled without `-C
metadata`. This can result in naming conflicts which need to be resolved.

By passing `-C metadata` to the in-tree crates we ship it should add some extra
salt to all symbol names to ensure that they don't collide.