add is_xplat to env_interface and runtime wrapper (#2643)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2643

`fbcode/executorch/kernels/test/util.bzl` is included in shim now.

Remove the dependency on `fbsource//tools/build_defs:fbsource_utils.bzl`, so we can reference the file in OSS Cmake build.

Specifically, required for: https://github.com/pytorch/executorch/pull/2633

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Reviewed By: cccclai

Differential Revision: D55300977

fbshipit-source-id: ee0a2c49bc3ae3bf1198c85eba759c3969cb466f