rust: Kconfig: Factor out whether PL011 is Rust or C

rust: Kconfig: Factor out whether PL011 is Rust or C

Currently every board that uses the PL011 duplicates the logic that
selects the Rust implementation if Rust was enabled and the C
implementation if it does not.  Factor this out into a separate
Kconfig stanza, so that boards can go back to simply doing "select
PL011" and get whichever implementation is correct for the build.

This fixes a compilation failure if CONFIG_VMAPPLE is enabled
in a Rust build, because hw/vmapple/Kconfig didn't have the
"pick the Rust PL011 if Rust is enabled" logic in it.

Fixes: 59f4d65584bd33 ("hw/vmapple/vmapple: Add vmapple machine type")
Reported-by: Tanish Desai <tanishdesai37@gmail.com>
Analyzed-by: Tanish Desai <tanishdesai37@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20250319193110.1565578-2-peter.maydell@linaro.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>