spanification: Add #pragma allow_unsafe_buffers to remaining Dawn files

As part of the effort to enforce -Wunsafe-buffer-usage globally in
Dawn, this CL transitions from coarse, GN-level directory
suppressions to targeted file-level `#pragma allow_unsafe_buffers`
exclusions for files that weren't detected by the script or were
modified since the last tool run.

This establishes a new baseline where newly added files, and
existing files without the pragma, are strictly checked for safe
buffer usage. This immediately prevents the introduction of new
unsafe patterns into the codebase from clean files.

Upcoming follow-ups for this effort include:
- Removing the top-level directory exclusions from
  unsafe_buffers_paths.txt.
- Transitioning from coarse file-level pragmas to fine-grained,
  instruction-level opt-outs (UNSAFE_TODO).

This CL has no behavior changes.

This patch was fully automated using:https://paste.googleplex.com/6662342478790656

For more context, see the internal doc:http://go/unsafe-buffer-dawn

Bug: 507078598, 512077495
Change-Id: I6c9b63d39978dc7b164f23a64b128e0c29115fff
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/307995
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>