rust: kbuild: define `procmacro-name` function

rust: kbuild: define `procmacro-name` function

Since we are adding one more proc macro crate (`zerocopy-derive`),
we are refactoring their handling.

Thus define a `procmacro-name` function and use it to fill the existing
variables' values.

Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://patch.msgid.link/20260608141439.182634-5-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>