Change build files to treat NNBD fork as a build mode

[munificent]

Currently, the in-progress support for building tools using the forked NNBD SDK uses a separate set of build targets that output next to the non-NNBD built stuff. This requires a lot of duplicate build steps and careful edits to avoid stepping on the non-forked output files in the output directory.

Instead, a simpler option is probably to treat NNBD mode as another parameter the way architecture and mode are handled. When you pass --nnbd to build.py, it will create a separate xcodebuild/ReleaseX64NNBD directory (or whatever, for the other modes and arches) and put everything there.

This means we don't need to duplicate all of the build targets. It does mean we need to change build.py and gn.py to plumb the NNBD stuff through.

[munificent]

Landed in https://dart-review.googlesource.com/c/sdk/+/119602.

There is now a is_nnbd flag you can use inside the GN files to determine whether an implementation should use the forked NNBD SDK.