[build] use cross-compile to build macOS x86_64 target for WebGPU (#25617)
### Description
use cross-compile to build x86_64 target for WebGPU. This is required
for incoming Dawn upgrade
(https://github.com/microsoft/onnxruntime/pull/25461). Latest dawn need
at least Clang v16 to compile, but available macOS in CI pipelines are
up to v15.2.
This PR depends on #25615.
### Motivation and Context
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