meson: Only allow CONFIG_USER_ONLY from certain source sets

meson: Only allow CONFIG_USER_ONLY from certain source sets

Poison CONFIG_USER_ONLY and CONFIG_SOFTMMU unless
the compilation unit is in specific_ss, libuser_ss,
or libsystem_ss.  This is intended to prevent files
being incorrectly added to common_ss.

Remove #ifndef CONFIG_USER_ONLY / #error / #endif blocks.
All they do is trigger the poison error.

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>