When -d cpu logging was handled by target-foo/translate.c,
it was controled by DEBUG_DISAS, which is enabled by default.
Use the same condition in cpu_exec.

At the same time, reduce the if-deffery by assuming no flags
update is required for the target.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>