cmd/compile: align arm64 stack frames correctly

If the compiler doesn't do it, cmd/internal/obj/arm64 will,
and that will break the zeroing of ambiguously live values
done in zerorange, which in turn produces uninitialized
pointer cells that the GC trips over.

For #9880.

Change-Id: Ice97c30bc8b36d06b7b88d778d87fab8e1827fdc
Reviewed-on: https://go-review.googlesource.com/12847
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/compile/internal/arm64/ggen.go

@@ -19,6 +19,13 @@ func defframe(ptxt *obj.Prog) {
 
 	ptxt.To.Val = int32(gc.Rnd(gc.Curfn.Type.Argwid, int64(gc.Widthptr)))
 	frame := uint32(gc.Rnd(gc.Stksize+gc.Maxarg, int64(gc.Widthreg)))
+
+	// arm64 requires that the frame size (not counting saved LR)
+	// be empty or be 8 mod 16. If not, pad it.
+	if frame != 0 && frame%16 != 8 {
+		frame += 8
+	}
+
 	ptxt.To.Offset = int64(frame)
 
 	// insert code to zero ambiguously live variables