rwlock expects the struct thread to be aligned on 8 bytes, so make sure

thread0 is.

sys/kern/init_main.c

@@ -90,7 +90,7 @@ void mi_startup(void);				/* Should be elsewhere */
 static struct session session0;
 static struct pgrp pgrp0;
 struct	proc proc0;
-struct	thread thread0;
+struct	thread thread0 __aligned(8);
 struct	ksegrp ksegrp0;
 struct	vmspace vmspace0;
 struct	proc *initproc;