[dev.link] cmd/link: use new dodata on Plan 9 and Wasm

They don't have fancy Adddynrel stuff, so we can just enable it.

Change-Id: I84082c3187d8a9ffa3a9c5458959794df0e3c2b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/230030
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/link/internal/ld/main.go

@@ -211,6 +211,8 @@ func Main(arch *sys.Arch, theArch Arch) {
 			if !ctxt.IsAMD64() {
 				*flagnewDoData = false
 			}
+		case ctxt.IsPlan9(), ctxt.IsWasm():
+			// supported
 		default:
 			*flagnewDoData = false
 		}