rust: kbuild: support using libraries in `rustc_procmacro`

rust: kbuild: support using libraries in `rustc_procmacro`

Proc macros such as `macros` and `pin-init` will need the ability to use
libraries such as `syn` (added later) in the `rustc_procmacro` command.

Thus add the support for it.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Gary Guo <gary@garyguo.net>
Tested-by: Jesung Yang <y.j3ms.n@gmail.com>
Link: https://patch.msgid.link/20251124151837.2184382-6-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>