The libcxxabi expression had a `standalone` argument in LLVM 7 which
triggered passing of the `-DLLVM_ENABLE_LIBCXX=ON` flag. In LLVM 11 this
flag and others are toggled by the `useLLVM` attribute of
`stdenv.hostPlatform` but this toggles a flag we don't need on Darwin.
Similar to the refactoring of LLVM 7 libc++abi by Ericson2314 in
3af7e98470, I have reintroduced the standalone argument for use in
bootstrapping the Darwin stdenv.
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
