llvm_38: update hashes and fix on darwin

[acowley]

Things done

• Built on platform(s)
  • NixOS
  • OS X
  • Linux

---

The hashes for libc++ and libc++abi were wrong.

There was also an incompatibility with nixpkgs on darwin which is now
weakly worked around: the "os_trace" macro changed definition in the OS
X development SDK since version 10.9 as used by nixpkgs. LLVM 3.8 uses
the new version, which I am temporarily replacing with a printf on
darwin as it is only used in one minor location.

Closes #14542

[mention-bot]

By analyzing the blame information on this pull request, we identified @andrewrk to be a potential reviewer

[andrewrk]

Looks good to me.

[vcunat]

overrideDerivation is (said to) better be avoided in default code. Why not put that conditional phase directly into the source?

[acowley]

I did it this way because it seems to only be a problem for 3.8. Would putting a self version check guarding that patch in the generic llvm.nix be better?

[vcunat]

Yes, but note there is no generic llvm.nix (it would be nice to have IMO) – the whole directory is copied every time.

[acowley]

Thank you for your patience, @vcunat! I was confused by quickly navigating to the file and inferred too much from its name. I've removed the use of overrideDerivation.

[joachifm]

To be clear, the hashes didn't change, but were always incorrect due to being copied from the 3.7 expression, right?

[vcunat]

Yes.