[recipes] Compute nanobench flags in gen_tasks

Change-Id: Ia37e0b9cd2f6b6c6e9a728741bec642cdba9f7b1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/274555
Commit-Queue: Eric Boren <borenet@google.com>
Reviewed-by: Ben Wagner aka dogben <benjaminwagner@google.com>
27 files changed