llvmPackages_12.lld: make sure llvm libunwind is passed

lld needs LLVM's libunwind for its headers. That libunwind is not part
of the tools scope in pkgs/development/compilers/llvm/12/default.nix,
which means that lld previously received libunwind from top-level pkgs
which of course doesn't have the required headers.

To resolve this pass libunwind from the libraries scope — platform
concerns don't really mattern as only libunwind.src is used.

libunwind was initially passed correctly, but that was removed in
e830db4320. This regression was likely
introduced accidentally.

pkgs/development/compilers/llvm/12/default.nix

@@ -96,6 +96,7 @@ let
 
     lld = callPackage ./lld {
       inherit llvm_meta;
+      inherit (libraries) libunwind;
     };
 
     lldb = callPackage ./lldb {