bind: fallback to const.Value.String for pre go1.6

Fixes golang/go#14615

Change-Id: I75e130e5b7b2534660098907fa1f044390c01d01
Reviewed-on: https://go-review.googlesource.com/20164
Reviewed-by: Elias Naur <elias.naur@gmail.com>

bind/genjava.go

@@ -535,7 +535,7 @@ func (g *javaGen) genConst(o *types.Const) {
 	// TODO(hyangah): should const names use upper cases + "_"?
 	// TODO(hyangah): check invalid names.
 	jType := g.javaType(o.Type())
-	val := o.Val().ExactString()
+	val := constExactString(o)
 	switch b := o.Type().(*types.Basic); b.Kind() {
 	case types.Int64, types.UntypedInt:
 		i, exact := constant.Int64Val(o.Val())