[kernel][thread] Allow pinning threads to a specific cpu

The existing runqueues are scanned on resched until a thread that is
not pinned to another cpu is found.

Per-pu runqueues can be added later to avoid this scan.

Change-Id: Idd7dbddb4b79ef38a0ccd0c42b53f8867020e6cd

Conflicts:
	kernel/thread.c

top/main.c

@@ -110,6 +110,7 @@ void lk_main(ulong arg0, ulong arg1, ulong arg2, ulong arg3)
 	// create a thread to complete system initialization
 	dprintf(SPEW, "creating bootstrap completion thread\n");
 	thread_t *t = thread_create("bootstrap2", &bootstrap2, NULL, DEFAULT_PRIORITY, DEFAULT_STACK_SIZE);
+	t->pinned_cpu = 0;
 	thread_detach(t);
 	thread_resume(t);