NaCl: Allow building Chromium for ARM64 with "enable_nacl = true"

This is an initial step to enable NaCl to work in ARM64 Chromium using
ARM32 NaCl.  This disables building the parts of NaCl that would need
to be built as ARM32 (including nacl_helper and various untrusted
nexes).  It enables building NaCl support into the ARM64 chrome
executable.

With this change, various NaCl tests pass when run on ARM64 Ubuntu
(20.04), including this test:

  xvfb-run -a ./out/default/browser_tests --gtest_filter=PPAPINaClNewlibTest.PostMessage

That requires that the following files are copied from an ARM32 build
of Chromium:

  nacl_helper
  nacl_helper_bootstrap
  nacl_irt_arm.nexe
  ppapi_nacl_tests_newlib.nmf
  ppapi_nacl_tests_newlib_arm.nexe

Two changes will be needed after this:

 * Building NaCl trusted code as ARM32 in an ARM64 build
 * Building NaCl untrusted code (nexes) as ARM32 in an ARM64 build

Landing this change first allows those to be more easily implemented
and tested in either order.

Bug: 1299021
Change-Id: Ib25ebbe56c91ed735beb87b0b3479b8b7621b389
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3430407
Reviewed-by: Derek Schuff <dschuff@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Mark Seaborn <mseaborn@chromium.org>
Cr-Commit-Position: refs/heads/main@{#984490}
NOKEYCHECK=True
GitOrigin-RevId: 7c29eff9686ae821b0a9c43980db75a83397c3aa
3 files changed