generate: fail if system isn't Linux x86_64

Currently the script will fail anyway (at the protoc lines) if the
system isn't of that architecture, so we might as well make it clear
why that failure would occur.

BUG=chromium:1174238
TEST=local run

Change-Id: I1fd7fb22fc4774efb6a6fa55b18d80e9479473bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2676167
Auto-Submit: Sean Abraham <seanabraham@chromium.org>
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
Reviewed-by: LaMont Jones <lamontjones@chromium.org>
1 file changed