translate-all: define and use DEBUG_TB_FLUSH_GATE

This gets rid of some ifdef checks while ensuring that the debug code
is compiled, which prevents bit rot.

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>
