eclass/arc-build: Enable multilib support

This patch modifies the arc-build eclass to support multilib sysroots,
which need overriding different variables and suffixing dependencies
with ABI USE flags.

BUG=b:36540057
TEST=cbuildbot --remote reef-release kevin-release elm-release
CQ-DEPEND=CL:*409892

Change-Id: Ia8473c50cda1cbb5bf293123a796c44a2b7bd0ef
Reviewed-on: https://chromium-review.googlesource.com/488744
Commit-Ready: Tomasz Figa <tfiga@chromium.org>
Tested-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed