Reland "Refactor target cpu arch flags into a gni file"

This reverts commit d7b7496caaee081eb97b03579c4cc7babdf5173d.

Reason for revert: Guarded with is_chromeos to prevent assertion failure

Original change's description:
> Revert "Refactor target cpu arch flags into a gni file"
>
> This reverts commit ec7cacd50e965d96dffe9c3cf87c220a1887db1b.
>
> Reason for revert:
> LUCI Bisection has identified this change as the culprit of a build failure. See the analysis: https://ci.chromium.org/ui/p/chromium/bisection/compile-analysis/b/8683043860166898161
>
> Sample failed build: https://ci.chromium.org/b/8683043860166898161
>
> If this is a false positive, please report it at http://b.corp.google.com/createIssue?component=1199205&description=Analysis%3A+https%3A%2F%2Fchromium-review.googlesource.com%2Fc%2Fchromium%2Fsrc%2F%2B%2F7805571&format=PLAIN&priority=P3&title=Wrongly+blamed+https%3A%2F%2Fci.chromium.org%2Fui%2Fp%2Fchromium%2Fbisection%2Fcompile-analysis%2Fb%2F8683043860166898161&type=BUG
>
> Original change's description:
> > 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}
> >
>
> Bug: 500845363
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Change-Id: I5a173cd3e587596348040bcc9b0de1550d77a0ec
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7808910
> Commit-Queue: luci-bisection@appspot.gserviceaccount.com <luci-bisection@appspot.gserviceaccount.com>
> Owners-Override: luci-bisection@appspot.gserviceaccount.com <luci-bisection@appspot.gserviceaccount.com>
> Bot-Commit: luci-bisection@appspot.gserviceaccount.com <luci-bisection@appspot.gserviceaccount.com>
> Cr-Commit-Position: refs/heads/main@{#1623672}

Bug: 500845363
Change-Id: I944e528f68f9ea946fb7f7bf31ec18cdc09f78a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7807772
Commit-Queue: Matt Stark <msta@google.com>
Reviewed-by: Junji Watanabe <jwata@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Matt Stark <msta@google.com>
Cr-Commit-Position: refs/heads/main@{#1626743}
NOKEYCHECK=True
GitOrigin-RevId: d87ba1f5a3056a26cddcdefbfe44d9c9f84f5ae7