update_chroot: only update llvm when binpkgs are available

BUG=chromium:642813
TEST=uprev llvm and run cros_sdk --delete and cros_sdk
     llvm was not updated.

Change-Id: Ic33a0ad96695516055ec0336ef6ed4f994934907
Reviewed-on: https://chromium-review.googlesource.com/380326
Commit-Ready: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/update_chroot b/update_chroot
index 333a03f..d810bef 100755
--- a/update_chroot
+++ b/update_chroot
@@ -98,7 +98,9 @@
   # a successful build.
   EMERGE_FLAGS+=" --useoldpkg-atoms=dev-lang/go"
   EMERGE_FLAGS+=" --useoldpkg-atoms=sys-devel/binutils"
+  EMERGE_FLAGS+=" --useoldpkg-atoms=sys-devel/clang"
   EMERGE_FLAGS+=" --useoldpkg-atoms=sys-devel/gcc"
+  EMERGE_FLAGS+=" --useoldpkg-atoms=sys-devel/llvm"
   EMERGE_FLAGS+=" --useoldpkg-atoms=sys-libs/glibc"
 fi