Make --print KIND=PATH unstable

https://github.com/rust-lang/rust/pull/113780 should have gone through
an MCP+FCP but wasn't, but instead of reverting the original PR, this PR
just make that new option unstable.

compiler/rustc_session/src/config.rs

@@ -2151,6 +2151,12 @@ fn collect_print_requests(
     prints.extend(matches.opt_strs("print").into_iter().map(|req| {
         let (req, out) = split_out_file_name(&req);
 
+        if out.is_some() && !unstable_opts.unstable_options {
+            handler.early_error(
+                "the `-Z unstable-options` flag must also be passed to \
+                 enable the path print option",
+            );
+        }
         let kind = match PRINT_KINDS.iter().find(|&&(name, _)| name == req) {
             Some((_, PrintKind::TargetSpec)) => {
                 if unstable_opts.unstable_options {