* [gn] Adds a flag to turn off git hash versioning
Without this flag, it is excessively tedious to build Dart SDK
for local testing on Fuchsia, since Fuchsia runner will reject
a Dart SDK without a matching version.
* [gn] Add flag --no-dart-version-git-info
This flag suppresses the SDK stamping by git hash, which allows
deploying self-built Dart SDK. Without this flag, it is impossible
to build and deploy a development Dart SDK on Fuchsia.