Simplify way we tag unreachable code with only ISC_UNREACHABLE()

Previously, the unreachable code paths would have to be tagged with:

    INSIST(0);
    ISC_UNREACHABLE();

There was also older parts of the code that used comment annotation:

    /* NOTREACHED */

Unify the handling of unreachable code paths to just use:

    UNREACHABLE();

The UNREACHABLE() macro now asserts when reached and also uses
__builtin_unreachable(); when such builtin is available in the compiler.

lib/isc/timer.c

@@ -118,7 +118,7 @@ schedule(isc_timer_t *timer, isc_time_t *now, bool signal_ok) {
 		break;
 	default:
 		INSIST(0);
-		ISC_UNREACHABLE();
+		UNREACHABLE();
 	}
 
 	/*
@@ -426,7 +426,7 @@ dispatch(isc_timermgr_t *manager, isc_time_t *now) {
 			break;
 		default:
 			INSIST(0);
-			ISC_UNREACHABLE();
+			UNREACHABLE();
 		}
 
 		timer->index = 0;