Add libLLVM 6.0.1 for use in libjulia 1.3.1

[fingolfin]

No description provided.

[fingolfin]

OK, I got

ERROR: LoadError: You must lock an extracted LLVM build to a particular LLVM_full build number!

and it is clear how to proceed. However, part of the issue was that the existing LLVM_jll 6.0.1 wasn't usable on PowerPC (see #1795 (comment)) so I assume I'd first have to rebuild the full LLVM_jll for 6.0.1. See #1915 for that.

[vchuravy]

We tend to use LLVM_full for the builds, no? libLLVM is a future addition.

[fingolfin]

Yes, but I only realized that after I made this PR and read the errors, which is why I then opened PR #1915 to deal with LLVM_full :-). Once (if?) that is merged, this PR here could be updated. Of course if you prefer, we can also close it and I'll resubmit once (if) PR #1915 is done.