tools: ynl: make the attr and msg helpers more C++ friendly

Folks working on a C++ codegen would like to reuse the attribute
helpers directly. Add the few necessary casts, it's not too ugly.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20240529192031.3785761-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>