Align stacks of kernel threads correctly at 16-byte boundaries rather than

making sure they are all misaligned at +8 bytes. This fixes clang builds
of powerpc64 kernels (aside from a required increase in KSTACK_PAGES which
will come later).

This commit from FreeBSD/powerpc64 with a clang-built kernel.

MFC after:	2 weeks

sys/powerpc/aim/vm_machdep.c

@@ -187,6 +187,7 @@ cpu_fork(struct thread *td1, struct proc *p2, struct thread *td2, int flags)
 	cf->cf_arg1 = (register_t)tf;
 
 	pcb->pcb_sp = (register_t)cf;
+	KASSERT(pcb->pcb_sp % 16 == 0, ("stack misaligned"));
 	#ifdef __powerpc64__
 	pcb->pcb_lr = ((register_t *)fork_trampoline)[0];
 	pcb->pcb_toc = ((register_t *)fork_trampoline)[1];

sys/powerpc/include/frame.h

@@ -94,6 +94,7 @@ struct callframe {
 	register_t	cf_func;
 	register_t	cf_arg0;
 	register_t	cf_arg1;
+	register_t	_padding;	/* Maintain 16-byte alignment */
 };
 #else
 struct callframe {