Standalone sample doesn't use libplatform for default platform
implementation. This is needed for Oilpan GitHub mirror, which won't
contain libplatform.

Bug: v8:10724
Change-Id: I2e20ad157263a5073d0ba9ae8a2e211b2fcb35ed
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2310362Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69016}