meson: merge lib{system, user}_ss with {system, user}_ss

meson: merge lib{system, user}_ss with {system, user}_ss

Now that target configuration can be applied to lib{system, user}_ss,
there is no reason to keep that separate from the existing {system,
user}_ss.
The only difference is that we'll now compile those files with
-DCOMPILING_SYSTEM_VS_USER, which removes poison for
CONFIG_USER_ONLY and CONFIG_SOFTMMU, without any other side effect.

We extract existing system/user code common common libraries to
lib{system, user}.

To not break existing meson files, we alias libsystem_ss to system_ss
and libuser_ss to user_ss, so we can do the cleanup in next commit.

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20250521223414.248276-6-pierrick.bouvier@linaro.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>