Add NGA compilation stubs

Change-Id: I5309005146b8121528ad23589fa64cc6bf286aee
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/402578
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
12 files changed