zig objcopy: support the compiler protocol

This commit extracts out server code into src/Server.zig and uses it
both in the main CLI as well as `zig objcopy`.

std.Build.ObjCopyStep now adds `--listen=-` to the CLI for `zig objcopy`
and observes the protocol for progress and other kinds of integrations.

This fixes the last two test failures of this branch when I run
`zig build test` locally.

lib/std/Build/ObjCopyStep.zig

@@ -113,6 +113,8 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
     };
 
     try argv.appendSlice(&.{ full_src_path, full_dest_path });
+
+    try argv.append("--listen=-");
     _ = try step.evalZigProcess(argv.items, prog_node);
 
     self.output_file.path = full_dest_path;