Refactor target cpu arch flags into a gni file

This should make it easy to pass into unified modules

Bug: 500845363
Link: https://chromium-review.googlesource.com/id/I7fb9a27664c029534e7b7b593fa8941e6a6a6964
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7805571
Reviewed-by: Junji Watanabe <jwata@google.com>
Auto-Submit: Matt Stark <msta@google.com>
Commit-Queue: Junji Watanabe <jwata@google.com>
Cr-Commit-Position: refs/heads/main@{#1623666}
NOKEYCHECK=True
GitOrigin-RevId: ec7cacd50e965d96dffe9c3cf87c220a1887db1b