Pass "-Wl,-undefined=dynamic_lookup" for dynamic libraries on darwin.

This linker flag causes undefined symbols to be ignored, which is the
default behavior on linux but not macos. This is required when building
shared libraries to use as e.g. Python modules. This also matches what
Bazel's built-in macos cc toolchain does.

toolchain/private/defs.bzl

@@ -54,7 +54,7 @@ def _target_darwin(gocpu, zigcpu):
             "libc/include/any-macos-any",
         ],
         linkopts = [],
-        dynamic_library_linkopts = [],
+        dynamic_library_linkopts = ["-Wl,-undefined=dynamic_lookup"],
         copts = [],
         bazel_target_cpu = "darwin",
         constraint_values = [