system: Move most files to libsystem_ss

system: Move most files to libsystem_ss

Some of the headers used require CONFIG_USER_ONLY.
Do not move vl.c, because it has other include dependencies
that are present in system_ss.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>