While building the chroot from source on the chromiumos-sdk builders, gcc gets emerged by cros_setup_toolchains.  It pretty consistently fails and then succeeds on a retry.  Example run:

https://luci-milo.appspot.com/buildbot/chromiumos/chromiumos-sdk/8373

This looks similar to the problems found in  crbug.com/762283 .  From looking at the logs, I suspect the concurrent binutils emerge may be interacting badly with the gcc build.