cmd/dist: copy cmd/internal/xcoff to bootstrap directory

Fixes build.

Change-Id: Id3d78ae9e4669371b4ebf29f69217d0809ffb829
Reviewed-on: https://go-review.googlesource.com/c/143917
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/buildtool.go

@@ -65,6 +65,7 @@ var bootstrapDirs = []string{
 	"cmd/internal/obj/wasm",
 	"cmd/internal/src",
 	"cmd/internal/sys",
+	"cmd/internal/xcoff",
 	"cmd/link",
 	"cmd/link/internal/amd64",
 	"cmd/link/internal/arm",