Get rid of most @rpath nonsense on Darwin

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.

pkgs/development/tools/textql/default.nix

@@ -16,10 +16,6 @@ buildGoPackage rec {
 
   goDeps = ./deps.nix;
 
-  preFixup = stdenv.lib.optionalString stdenv.isDarwin ''
-    install_name_tool -delete_rpath $out/lib $bin/bin/textql
-  '';
-
   meta = with stdenv.lib; {
     description = "Execute SQL against structured text like CSV or TSV";
     homepage = https://github.com/dinedal/textql;