This changes CPU check to use 'target_cpu' instead of 'v8_target_cpu'.

Bug: chromium:694255
Change-Id: Ic3ad5253e4e0b66b13e9f16a5842bcf49881fa52
Reviewed-on: https://chromium-review.googlesource.com/677994Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48119}