hw/*: Build stubs once

Move stubs to the global stub_ss[] source set. These files
are now built once for all binaries, instead of one time
per system binary.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20260225035739.42848-10-philmd@linaro.org>
7 files changed