boot/atari_m68k: fix stack after toscallLLWWWWW

Change-Id: I3c0712d2eee173eb1744b7eb8b5b35a3b4bdc71f
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6946
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

src/system/boot/platform/atari_m68k/toscalls.h

@@ -203,7 +203,7 @@ extern "C" {
 		move.l	%1,-(%%sp) \n			\
 		move.w	%[calln],-(%%sp)\n		\
 		trap	%[trapn]\n			\
-		add.l	#18,%%sp \n "			\
+		add.l	#20,%%sp \n "			\
 	: "=r"(retvalue)	/* output */		\
 	: "r"(_p1), "r"(_p2),				\
 	  "r"(_p3), "r"(_p4),				\