tool/gocross: make gocross opt-in instead of opt-out

gocross is not needed like it used to be, now that Go does
version stamping itself.

We keep it for the xcode and Windows builds for now.

This simplifies things in the build, especially with upcoming build
system updates.

Updates tailscale/corp#28679
Updates tailscale/corp#26717

Change-Id: Ib4bebe6f50f3b9c3d6cd27323fca603e3dfb43cc
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

version/print.go

@@ -20,6 +20,7 @@ var stringLazy = sync.OnceValue(func() string {
 	if gitCommit() != "" {
 		fmt.Fprintf(&ret, "  tailscale commit: %s%s\n", gitCommit(), dirtyString())
 	}
+	fmt.Fprintf(&ret, "  long version: %s\n", Long())
 	if extraGitCommitStamp != "" {
 		fmt.Fprintf(&ret, "  other commit: %s\n", extraGitCommitStamp)
 	}