Auto merge of #17638 - rillian:mac-bindgen, r=emilio,gps

Fix build_gecko.rs for cross-macOS builds.

This was [reviewed](https://bugzilla.mozilla.org/show_bug.cgi?id=1368083) over on the gecko side, and is now ready to land. This adds an additional build-time `bindgen.toml` file whose compiler args are merged into the libclang invocation.

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