tcg: Formalize tcg_out_mb

tcg: Formalize tcg_out_mb

Most tcg backends already have a function for this;
the rest can split one out from tcg_out_op.
Call it directly from tcg_gen_code.

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>