Auto merge of #14244 - upsuper:buildtime-bindgen, r=emilio

Use build script to generate binding files

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This is a WIP patch to make rust-bindgen a build time dependency of style component for stylo. This should make things more automatic. I convert majority of `regen.py` to `build_gecko.rs`.

It currently doesn't work on Windows, because of servo/rust-bindgen#271 (it works when I fix the generated file manually, though). I haven't tested other platforms.

It would break servo's CI for geckolib, because:
1. it needs libclang (which is probably easy to solve)
2. it needs `MOZ_OBJDIR` to be passed in so that it can generate bindings

@Manishearth @emilio Do you agree with this change? Do you have suggestion for the issues above?

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- [ ] These changes fix #__ (github issue number if applicable).

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