llvm 5: split out compiler-rt and remove libcxxabi dep

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.

pkgs/test/cc-wrapper/default.nix

@@ -3,7 +3,7 @@ with stdenv.lib;
 let
   # Sanitizers are not supported on Darwin.
   # Sanitizer headers aren't available in older libc++ stdenvs due to a bug
-  sanitizersBroken = stdenv.cc.isClang && versionOlder (getVersion stdenv.cc.name) "6.0.0";
+  sanitizersBroken = stdenv.cc.isClang && versionOlder (getVersion stdenv.cc.name) "5.0.0";
 in stdenv.mkDerivation {
   name = "cc-wrapper-test";