rust: kbuild: define `procmacro-extension` variable

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

Thus, instead of using `libmacros_extension` as the common variable to
hold the extension for all of them, use a dedicated variable with a more
generic name (including for its implementation).

Link: https://patch.msgid.link/20260608141439.182634-6-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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