The current hook specifies the path to the framework library, but
nixpkgs does not actually provide the library when linking against the
system framework. It provides a text-based stub (`.tbd`) instead. ld64
will find the stub and use it, but lld will not when the full path is
specified. Both linkers work if the extension is included, so do that
for compatibility with both. This fixes using lld with CoreFoundation
(e.g., to support LTO on Darwin).
8e56a2635f