semihosting: Move user-only implementation out-of-line

semihosting: Move user-only implementation out-of-line

Avoid testing CONFIG_USER_ONLY in semihost.h.
The only function that's required is semihosting_enabled.

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>