semihosting: Assert is_user in user-only semihosting_enabled

Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
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>

semihosting/user.c

@@ -11,6 +11,7 @@
 
 bool semihosting_enabled(bool is_user)
 {
+    assert(is_user);
     return true;
 }