tcg: Add tcg_out_tb_start backend hook

tcg: Add tcg_out_tb_start backend hook

This hook may emit code at the beginning of the TB.

Suggested-by: Jordan Niethe <jniethe5@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>