To be able to do LTO with clang we need the LTO-capable llvm-ar from
libllvm.out (== llvmPackages.llvm) which contains a superset of the llvm
bintools. This used to be passed through from stdenv.cc.cc as llvm, but
has been renamed to libllvm in 7869d16545.
llvm-ar would be in stdenv.cc.bintools.bintools provided that we are
using a clang stdenv which uses lld as its linker. In the future we could
thus make this inclusion conditional on stdenv.hostPlatform.linker.
Note that the build of foot.tests still fails, pending resolution of #123361.
631cbc3de7