There are bots on the chromium.lkgr waterfall that need to
be able to set v8_target_arch to "arm" while the rest of
the target build is "x86". This CL makes that argument
a declare_arg(), so that we can do that.

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

Review-Url: https://codereview.chromium.org/2007843002
Cr-Commit-Position: refs/heads/master@{#36464}