Since #420382, we set an absolute default CLANG_RESOURCE_DIR for LLVM
versions >= 21, which breaks the bootstrap tools because the absolute
path was removed by nuke-ref while building and isn't there while
bootstrapping.
Pass -resource-dir explicitly while testing Clang to make it work.
- Drop libSystem. It’s no longer needed. The SDK can be downloaded and
built with existing tools.
- Add jq and tapi. Adding these allows the stdenv bootstrap to stop
special-casing stage 0.
- Update tests for updated ld64. It handles code-signing properly, so
the signatures aren’t broken.