rust: kbuild: auto generate helper exports

This removes the need to explicitly export all symbols.

Generate helper exports similarly to what's currently done for Rust
crates. These helpers are exclusively called from within Rust code and
therefore can be treated similar as other Rust symbols.

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Tested-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20240817165302.3852499-1-gary@garyguo.net


[ Fixed dependency path, reworded slightly, edited comment a bit and
  rebased on top of the changes made when applying Andreas' patch
  (e.g. no `README.md` anymore, so moved the edits).  - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>