This enables the v8_enable_embedded_builtins gn flag on non-ia32 builds
and adds a new --stress-off-heap-code test mode to fyi bots.

v8_enable_embedded_builtins=true changes accesses to constants and
external references to go through the root list in builtins code.

--stress-off-heap-code copies builtins code off-heap on isolate
creation.

A few drive-by-fixes:
- ensure that we actually inspect the correct builtin during
  isolate-independence testing.
- relax tests to decrease maintenance (now we only fail if a builtin
  should be isolate-independent but isn't).
- switch to a different off-heap-trampoline register on arm due to
  conflicts with custom stub linkages.

Cq-Include-Trybots: luci.v8.try:v8_linux64_fyi_rel_ng
Bug: v8:6666
Change-Id: I09ad3c75cb4342f4c548ea780f275993730896c8
Reviewed-on: https://chromium-review.googlesource.com/934281
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Hablich <hablich@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51717}