meson: Introduce top-level libuser_ss and libsystem_ss

meson: Introduce top-level libuser_ss and libsystem_ss

We already have two subdirectories for which we need
to build files twice, for user vs system modes.
Move this handling to the top level.

This cannot be combined with user_ss or system_ss,
because the formulation has not been extended to support
configuration symbols.

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