cmd/compile/internal/gc: make sure nonzero-sized objects have nonzero alignment

See issue #11354.

Change-Id: I6817d499e6c02e4a9c83f80bc66a2bee368a69ec
Reviewed-on: https://go-review.googlesource.com/11406
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/compile/internal/gc/align.go

@@ -119,6 +119,10 @@ func dowidth(t *Type) {
 	}
 
 	if t.Width > 0 {
+		if t.Align == 0 {
+			// See issue 11354
+			Fatal("zero alignment with nonzero size %v", t)
+		}
 		return
 	}