use machno consistently; drop use of "core"

Reference: /n/atom/patch/applied2013/toshmachno
Date: Sat Jun 15 06:00:06 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/include/tos.h	Sat Jun 15 06:00:06 2013
+++ /sys/include/tos.h	Sat Jun 15 06:00:06 2013
@@ -18,7 +18,7 @@
 	vlong	pcycles;	/* cycles spent in process (kernel + user) */
 	ulong	pid;		/* might as well put the pid here */
 	ulong	clock;
-	uint	core;		/* mp->machno */
+	uint	machno;		/* mp->machno */
 	/* top of stack is here */
 };