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