libtool will let any flags that follow -Xcompiler. This is probably easier instead of trying to fix individual packages many of which are in portage-stable.

Argh, GCC does not support this flag so packages still using GCC do not build e.g. vboot_reference.

Fixed Xcompiler flag issue in https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/1058110

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/365d04e7f11bd6684d46992a8806f1915a824304

commit 365d04e7f11bd6684d46992a8806f1915a824304
Author: Manoj Gupta <manojgupta@google.com>
Date: Wed May 16 12:08:38 2018

profile: Filter unsupported GCC flags with cros_use_gcc.

Filter some unsupported FLAGS when GCC is set as the compiler.
This includes sanitizer flags and "-Xcompiler" flag.

BUG= chromium:842923 
TEST=emerge-amd64-generic vboot_reference builds.

Change-Id: I9fe22c59621c4a7bf637af28a55e504cc206ff86
Reviewed-on: https://chromium-review.googlesource.com/1058561
Commit-Ready: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

[modify] https://crrev.com/365d04e7f11bd6684d46992a8806f1915a824304/profiles/base/profile.bashrc

normally this is what elibtoolize is for, but that sorry requires packages to opt-in to the call.  stale cached autotool outputs are a pita in general, and libtool isn't the only thing that suffers.

if you can pass via -Xcompiler, that is a better route for CrOS to take until Gentoo can find a common solution.