New hook to return true when an architectural breakpoint is
to be recognized and false when it should be suppressed.

First use must wait until other pieces are in place.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>