See the changes in BUILD.gn for an explanation of why a new flag is necessary.
It's likely that current usages of V8_TARGET_OS_* and V8_TARGET_ARCH_* also need
to change, but this is good enough for now to ensure both Win/cross/x86 and
Win/cross/x64 build.

BUG=chromium:945659
R=machenbach,thakis
TBR=jgruber

Change-Id: Ie2765db91a1c0d8c72ccf42c9d7fece792d9b252
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1542500
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60687}