[recipes] Move DM flags to a separate file

This particular location was chosen because we can rely on it being
present in the recipe bundle. We'll soon run the script to generate
DM flags in gen_tasks, before finally porting the logic to Go inside
gen_tasks itself.

Change-Id: I36ec481cdabffe8d1da662cc813dda43fe7b21f8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/274053
Commit-Queue: Eric Boren <borenet@google.com>
Reviewed-by: Ben Wagner aka dogben <benjaminwagner@google.com>

Author: erock2112