translate-all: define and use DEBUG_TB_CHECK_GATE

This prevents bit rot by ensuring the debug code is compiled when
building a user-mode target.

Unfortunately the helpers are user-mode-only so we cannot fully
get rid of the ifdef checks. Add a comment to explain this.

Suggested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>