mk was failing to read archive times because sizeof(struct ar_hdr) == 64
on amd64 (8-byte alignment), and not 60.  this caused the read to be too
large, and time timestamps to all be set to 0.

Reference: /n/sources/patch/applied/amd64-mk-ar
Date: Fri Jan 11 01:21:17 CET 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/mk/archive.c	Fri Jan 11 01:18:59 2013
+++ /sys/src/cmd/mk/archive.c	Fri Jan 11 01:18:57 2013
@@ -101,7 +101,7 @@
 	}
 	at = d->mtime;
 	free(d);
-	while(read(fd, (char *)&h, sizeof(h)) == sizeof(h)){
+	while(read(fd, (char *)&h, SAR_HDR) == SAR_HDR){
 		t = atol(h.date);
 		if(t >= at)	/* new things in old archives confuses mk */
 			t = at-1;