target/arm: Build cpu32-system.o as common object

cpu32.c only contains CPU types used in 32-bit system emulation:
rename it as cpu32-system.c; always compile the file but only
register the QOM types for the 32-bit binary.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com>
Acked-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20260526203722.79463-17-philmd@linaro.org>