meson: add common libs for target and target_system

meson: add common libs for target and target_system

Following what we did for hw/, we need target specific common libraries
for target. We need 2 different libraries:
- code common to a base architecture
- system code common to a base architecture

For user code, it can stay compiled per target for now.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-id: 20250512180502.2395029-4-pierrick.bouvier@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>