Create Bazel Toolchain for running iOS builds

- Get ` //tools/window ` building with the toolchain.

Change-Id: I93a460bbae895ab90f28ddd06d81b6e2f640650a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/894496
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Ashwin Verleker <ashwinpv@google.com>
14 files changed