m68k: fix trigraph ignored warning in setox.S

Fix the warning: trigraph ??/ ignored, use -trigraphs to enable
caused by the recent removal of -traditional option.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>