SerenityOS: Unset CMAKE_DL_LIBS because LibDl no longer exists

Serenity's LibDl was merged into LibC to simplify the build and port
infrastructure [1].  Set `CMAKE_DL_LIBS` to the empty string to match
what other platforms do.  Update the platform module added by
commit 45ca894164 (SerenityOS: Add Platform module, 2022-01-02,
v3.25.0-rc1~635^2).

[1] https://github.com/SerenityOS/serenity/pull/14854

Issue: #23589