linux32 is not supported now.

Bug: 931218
Change-Id: I753c8e1b0de5126e944d71cd7725e0ed8a4df5d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3609948
Auto-Submit: Junji Watanabe <jwata@google.com>
Commit-Queue: Junji Watanabe <jwata@google.com>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>