Add an "--nnbd" flag to build.py.

This treats whether or not to use the forked NNBD SDK as a build flag
similar to mode or architecture. When this flag is passed, it appends
"NNBD" to the main output directory, like xcodebuild/ReleaseX64NNBD.

It also defines a "use_nnbd" flag that is available inside BUILD.gn
files to determine whether or not to use the forked SDK. This flag
currently isn't used. Implementation teams will need to edit their
build steps to take that into account.

In order to use this mode on the bots, we'll need to tweak the bot
scripts to figure out whether the mode is enabled or not. There are a
couple of TODOs in the Python scripts for that. I'm not sure how that
should be specified in dart_sdk.py.

This CL does not remove my previous change to allow building the NNBD
fork next to dart-sdk in the same root output directory. I'll revert
those changes in a different CL once this one lands.

Change-Id: I3bd28f273106ee90caf9474b2fadad120b2c2d0b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/119602
Commit-Queue: Bob Nystrom <rnystrom@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>