macos stage3: add link support for system libc++

- activates when -DZIG_SHARED_LLVM=ON
- activates when llvm_config is used and --shared-mode is shared
- otherwise vendored libc++ is used

closes #23189

Author: mikdusan

build.zig

@@ -782,7 +782,17 @@ fn addCmakeCfgOptionsToExe(
                 mod.linkSystemLibrary("unwind", .{});
             },
             .ios, .macos, .watchos, .tvos, .visionos => {
-                mod.link_libcpp = true;
+                if (static or !std.zig.system.darwin.isSdkInstalled(b.allocator)) {
+                    mod.link_libcpp = true;
+                } else {
+                    // Avoid using `mod.linkSystemLibrary()`, which:
+                    //   - is semantically equivalent to `-lc++`
+                    //   - and enables `mod.link_libcpp`
+                    // Instead, add the full object pathname.
+                    const sdk = std.zig.system.darwin.getSdk(b.allocator, b.graph.host.result) orelse return error.SdkDetectFailed;
+                    const libcxx = b.pathJoin(&.{ sdk, "usr/lib/libc++.tbd" });
+                    exe.root_module.addObjectFile(.{ .cwd_relative = libcxx });
+                }
             },
             .windows => {
                 if (target.abi != .msvc) mod.link_libcpp = true;