The latest plume sources need LLVM 8.0.0 for compilation. This PR bumps LLVM and restores arm64 builds.
arm32 builds should work but nix is currently broken for builds. See #611 for more information on the arm32 breakages. Once nix is updated, this should also fix arm32 builds.
I’ve also removed a pair of comments that reference unneeded LLVM tools.