Previously we only set these when using musl.
See https://github.com/NixOS/nixpkgs/pull/42452#discussion_r197618464
Same should be done for changes in #42452,
I'll add a follow-up commit doing so in the PR containing this
or wherever makes sense-- depending on how review/merging of that goes.
Sending similar changes for LLVM 4 to master, as suggested.
We already did them on non-mass-rebuild llvm 6. Also, this allows
simplifying the stdenv booting.
We were missing the libcxxabi dep in compile-rt in llvm 6, so fixed that
too.
This requires some small changes in the stdenv, then working around the
weird choice LLVM made to hardcode @rpath in its install name, and then
lets us remove a ton of annoying workaround hacks in many of our Go
packages. With any luck this will mean less hackery going forward.
