update libmp mkfile to try harder to clean up;
the solution is hacky.

Reference: /n/atom/patch/applied2013/mpmkfile
Date: Wed Jun 19 16:07:02 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libmp/mkfile	Wed Jun 19 16:06:31 2013
+++ /sys/src/libmp/mkfile	Wed Jun 19 16:06:32 2013
@@ -20,8 +20,9 @@
 		cd $i
 		mk $MKFLAGS $target
 	}
+	rm -f */*.[012456789kvx]	# in case build not in CPUS
 update:V:
-	for(i in $DIRS)@{
+	for(i in port $CPUS)@{
 		echo $i
 		cd $i
 		mk $MKFLAGS $target