[VM] Increase BigInt digit size from 16 to 32 bits.
[VM] Increase BigInt digit size from 16 to 32 bits.

Change-Id: I60e7e34559d11d80fde7955dec98b21b7029e199
Reviewed-on: https://dart-review.googlesource.com/40962
Commit-Queue: Régis Crelier <c2bf536a10d9f2530c38e4e8da517718be8c39ce@google.com>
Reviewed-by: Alexander Markov <239e774fd29abc1267878f57afe1a8a03e0ca887@google.com>

dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk