It is likely that CrOS and other distros will want to be
able to build the v8 snapshot with a custom toolchain.

This CL makes snapshot_toolchain a declared build arg, and
renames it to v8_snapshot_toolchain to minimize the risk
of a name collision.

R=machenbach@chromium.org, jochen@chromium.org
BUG=608596

Review-Url: https://codereview.chromium.org/1969693002
Cr-Commit-Position: refs/heads/master@{#36193}