The main thing was using `llvm_meta` in all versions.
Secondarily:
- libunwindx7: Forgot to split outputs
- libcxx{,abi} 12: Forgot to apply output-splitting patches.
- simplify `useLLVM` stdenv-switching logic.
- openmp always gets its own directory
This might be a bit debatable but upstream uses "xx" instead of "++"
when using it as identifier / in the code (file/directory names, build
scripts, website URLs, etc.) so we should probably too.
And at least the attribute name and pname will be consistent now.