acid files are arch-dependent.  use a$O not a.

Reference: /n/atom/patch/applied2013/upasmk
Date: Tue Jun 18 23:57:10 CES 2013
Signed-off-by: quanstro@quantsro.net

# rm /sys/src/cmd/upas/fs/seg

--- /sys/src/cmd/upas/fs/mkfile	Tue Jun 18 23:56:41 2013
+++ /sys/src/cmd/upas/fs/mkfile	Tue Jun 18 23:56:41 2013
@@ -36,7 +36,7 @@
 CFLAGS=$CFLAGS  -I../common
 
 acid:V:
-	$CC -a $CFLAGS fs.c>a
+	$CC -a $CFLAGS fs.c>a$O
 
 chkidx: mtree.$O chkidx.$O
 	$LD $LDFLAGS -o $target $prereq