exec: fix check_watchpoint exiting cpu_loop

In case of BP_STOP_BEFORE_ACCESS watchpoint check_watchpoint intends to
signal EXCP_DEBUG exception on exit from cpu loop, but later overwrites
exception code by the cpu_resume_from_signal call.

Use cpu_loop_exit with BP_STOP_BEFORE_ACCESS watchpoints.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>