This change makes  the architecture that we target generated
v8 code for a property of the current toolchain, rather than a
global setting that applies to every toolchain.

This will allow us to properly build two snapshots for two different
architectures in a single build, which is needed for android
webview/monochrome builds.

R=brettw@chromium.org, jochen@chromium.org, michaelbai@chromium.org
BUG=625383

Review-Url: https://codereview.chromium.org/2116913002
Cr-Commit-Position: refs/heads/master@{#37805}