mosys: allow openat on amd64 and arm.

This allows openat on amd64 and arm board because glibc is using this
system call when calling open.

BUG=chromium:894587
TEST=no crash when running /usr/sbin/mosys on samus with glibc 2.27

Change-Id: If0d0df8afdd433f50b0c9dc048e9e7823399bf69
Reviewed-on: https://chromium-review.googlesource.com/1277642
Commit-Ready: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
2 files changed