ANDROID: rust: allow MODVERSIONS by no longer exporting symbols

Rust symbols on the kernel interface don't support MODVERSIONS, but
users may want to use Rust only for built-in components. Specifically,
the new binder driver will be built-in and this allows us to enable
RUST without disabling MODVERSIONS to support that.

Work is still ongoing on the list to determine a form of MODVERSIONS
support that upstream will support. In the meantime, this patch stops
us from exporting Rust symbols to modules, so we won't accidentally
enable support for Rust modules in our kernel before MODVERSIONS
is working upstream.

This patch does not belong upstream as it is a local placeholder
to unblock deployment of the Rust binder driver until an upstream
solution can be decided on.

Link: https://lore.kernel.org/lkml/CAGSQo005hRiUZdeppCifDqG9zFDJRwahpBLE4x7-MyfJscn7tQ@mail.gmail.com/


Signed-off-by: Matthew Maurer <mmaurer@google.com>
Bug: 308623792
Change-Id: I6d5a27af73e64eb2b5b922c5ebb825c4e4b5b312