found with fdupes
```
pkgs/development/compilers/llvm/8/libcxxabi/no-threads.patch
pkgs/development/compilers/llvm/10/libcxxabi/no-threads.patch
pkgs/development/compilers/llvm/9/libcxxabi/no-threads.patch
pkgs/development/compilers/llvm/11/libcxxabi/no-threads.patch
```
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.
Always set `SRCTOP`, set it with abs path
llvmPackages: Bump minimum version for FreeBSD
llvmPackages_*, libgcc, compiler_rt: Hack in enough libs that one can compiler C
freebsd.compat: Rename some things to work around cc-wrapper change
0bea4a194f / #191724 in particular
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