native-xtools: avoid libllvm while populating the sysroot

Prior to 021385aba562, MK_CLANG=no was sufficient to avoid descending
into lib/clang, but the referenced change added a couple of other
enabling knobs. Turn those off, too, to continue avoiding libllvm.

With this change, we no longer end up with a libllvm using the wrong
default target triple; poudriere jail -cx works once again.

Reported by: bhughes, imp, probably others
Fixes: 021385aba562 ("Add WITH_LLVM_BINUTILS to install LLVM ...")