Fix the tests to work on GOARCH=386.
Also add another test suite that tests on GOARCH=386.
Change-Id: I41290998053dc059347adb8af70012dc5cc31811
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/241537
Reviewed-by: Herbie Ong <herbie@google.com>