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.

CMakeLists.txt

@@ -623,6 +623,7 @@ set(ZIG_STAGE2_SOURCES
     "${CMAKE_SOURCE_DIR}/src/print_targets.zig"
     "${CMAKE_SOURCE_DIR}/src/print_zir.zig"
     "${CMAKE_SOURCE_DIR}/src/register_manager.zig"
+    "${CMAKE_SOURCE_DIR}/src/Server.zig"
     "${CMAKE_SOURCE_DIR}/src/target.zig"
     "${CMAKE_SOURCE_DIR}/src/tracy.zig"
     "${CMAKE_SOURCE_DIR}/src/translate_c.zig"