tcg: Introduce tcg_use_softmmu

Begin disconnecting CONFIG_SOFTMMU from !CONFIG_USER_ONLY.
Introduce a variable which can be set at startup to select
one method or another for user-only.

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