Excludes //v8/gni/snapshot_toolchain.gni on iOS.

This fixes the arm64 iOS Simulator build.

BUG=1250511

Change-Id: If69d2d38b9918bd8fc2682fe676331146635b7a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3167170


Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/main@{#922634}