flatbuffers: upgraded package to upstream

Upgraded dev-libs/flatbuffers to version 1.12.0 on amd64, arm

BUG=b:162916911
TEST=cq passes

Change-Id: I09e84fd484ec94f3cb955dfeefbb10f48e76f987
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/2346565
Tested-by: Jim Pollock <jmpollock@chromium.org>
Auto-Submit: Jim Pollock <jmpollock@chromium.org>
Reviewed-by: Allen Webb <allenwebb@google.com>
Commit-Queue: Jim Pollock <jmpollock@chromium.org>
4 files changed