use the expected arch for configure for amd64

Reference: /n/atom/patch/applied/unameamd64
Date: Sun Aug 16 17:58:14 CES 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/src/ape/lib/ap/plan9/uname.c	Sun Aug 16 17:58:02 2015
+++ /sys/src/ape/lib/ap/plan9/uname.c	Sun Aug 16 17:58:03 2015
@@ -17,9 +17,11 @@
 	n->release = "4";			/* edition */
 	n->version = "0";
 	n->machine = getenv("cputype");
-	if(!n->machine)
+	if(n->machine == 0)
 		n->machine = "?";
-	if(strcmp(n->machine, "386") == 0)
+	else if(strcmp(n->machine, "386") == 0)
 		n->machine = "i386";		/* for gnu configure */
+	else if(strcmp(n->machine, "amd64") == 0)
+		n->machine = "x86_64";
 	return 0;
 }