Use native sign() in BYOND 516 (#20452)

https://www.byond.com/docs/ref/#/proc/sign

BYOND 516 adds `sign()` as a built-in proc.

Signed-off-by: ike709 <ike709@users.noreply.github.com>

code/__HELPERS/unsorted.dm

@@ -227,8 +227,10 @@ Turf and target are seperate in case you want to teleport some distance from a t
 			return 1
 	return 0
 
+#if DM_VERSION < 516
 /proc/sign(x)
 	return x!=0?x/abs(x):0
+#endif
 
 /proc/getline(atom/M,atom/N)//Ultra-Fast Bresenham Line-Drawing Algorithm
 	var/px=M.x		//starting x