cmd/compile: mark go.itab.* symbols local earlier

This feels a bit like a layering violation, but as the bug report shows
it is sometimes necessary.

Fixes #17642

Change-Id: I4ba060bb1ce73a527ce276e5a769c44692b50016
Reviewed-on: https://go-review.googlesource.com/32236
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

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

@@ -979,6 +979,7 @@ func itabname(t, itype *Type) *Node {
 		Fatalf("itabname(%v, %v)", t, itype)
 	}
 	s := Pkglookup(t.tconv(FmtLeft)+","+itype.tconv(FmtLeft), itabpkg)
+	Linksym(s).Set(obj.AttrLocal, true)
 	if s.Def == nil {
 		n := newname(s)
 		n.Type = Types[TUINT8]