This moves setting the v8_target_arch default to the shared
v8.gni, so that it's available in build_overrides for
stand-alone mode.

BUG=chromium:605732, chromium:474921
NOTRY=true

Review-Url: https://codereview.chromium.org/2006213002
Cr-Commit-Position: refs/heads/master@{#36465}