Make Bazel boilerplate presubmit platform dependent

This also adds the boilerplate to a few files using a bespoke script.

Change-Id: Ie65d2490ada73e08ae64d1757cc90a8c8eaa9e74
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1000396
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
10 files changed