rust: fix regexp in scripts/is_rust_module.sh

nm can use "R" or "r" to show read-only data sections, but
scripts/is_rust_module.sh can only recognize "r", so with some versions
of binutils it can fail to detect if a module is a Rust module or not.

Right now we're using this script only to determine if we need to skip
BTF generation (that is disabled globally if CONFIG_RUST is enabled),
but it's still nice to fix this script to do the proper job.

Moreover, with this patch applied I can also relax the constraint of
"RUST depends on !DEBUG_INFO_BTF" and build a kernel with Rust and BTF
enabled at the same time (of course BTF generation is still skipped for
Rust modules).

[ Miguel: The actual reason is likely to be a change on the Rust
  compiler between 1.61.0 and 1.62.0:

    echo '#[used] static S: () = ();' |
        rustup run 1.61.0 rustc --emit=obj --crate-type=lib - &&
        nm rust_out.o

    echo '#[used] static S: () = ();' |
        rustup run 1.62.0 rustc --emit=obj --crate-type=lib - &&
        nm rust_out.o

  Gives:

    0000000000000000 r _ZN8rust_out1S17h48027ce0da975467E
    0000000000000000 R _ZN8rust_out1S17h58e1f3d9c0e97cefE

  See https://godbolt.org/z/KE6jneoo4

. ]

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Reviewed-by: Eric Curtin <ecurtin@redhat.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>