meson: apply target config for picking files from lib{system, user}

meson: apply target config for picking files from lib{system, user}

semihosting code needs to be included only if CONFIG_SEMIHOSTING is set.
However, this is a target configuration, so we need to apply it to the
lib{system, user}_ss.

As well, this prepares merging lib{system, user}_ss with
{system, user}_ss.

Acked-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20250521223414.248276-5-pierrick.bouvier@linaro.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>