target/arm: Implement debug_check_breakpoint

Reuse the code at the bottom of helper_check_breakpoints,
which is what we currently call from *_tr_breakpoint_check.

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>