Add libcxx and libcxxabi to host toolchain packages.

libcxx and libcxxabi need to be installed for host package
compilation with clang + libc++.

BUG=chromium:724628
TEST=libcxx packages are pulled in when running cros_setup_toolchain

CQ-DEPEND=CL:595187

Change-Id: I662f7ff25f5851375a4a3b784db297b84e450019
Reviewed-on: https://chromium-review.googlesource.com/578137
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Trybot-Ready: Manoj Gupta <manojgupta@chromium.org>
Commit-Queue: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cros_setup_toolchains.py b/scripts/cros_setup_toolchains.py
index 667f8b5..288c4f9 100644
--- a/scripts/cros_setup_toolchains.py
+++ b/scripts/cros_setup_toolchains.py
@@ -103,6 +103,8 @@
   # crossdev.
   MANUAL_PKGS = {
       'llvm': 'sys-devel',
+      'libcxxabi': 'sys-libs',
+      'libcxx': 'sys-libs',
   }
 
   @classmethod