The Dart benchmarks will soon be forked for non-nullability and there
will be two implementations of the FfiBoringssl benchmark. This change
moves the native BoringSSL build a level up so it can be shared between
the two implementations, avoiding doubling the needed filesystem space.
Change-Id: I8bcc113e19715865f28bc1950cf07ba6cc7ea61f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/148760
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Jonas Termansen <sortie@google.com>
