FreeBSD doesn't use LLVM's cxxabi implementation, for backwards
compatibility reasons. Software expects the libcxxrt API when
building on FreeBSD. This fixes the build of
pkgsCross.x86_64-freebsd.boost.
Same adjustment as made for libc++abi in #185766, for the same reason:
the unamended dylib links to the libc++abi in the build stdenv, which
is the wrong version.
Tested on Darwin with LLVM 14 stdenv, but the phase is added to all
versions, including 11 - so this will cause a mass rebuild.
See: https://github.com/NixOS/nixpkgs/pull/185766
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
