Auto merge of #11925 - emilio:stylo-simplify-bindgen-setup, r=bholley

stylo: Simplify setup_bindgen.sh now that servo/rust-bindgen exists.

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The clang-3.8 instead of clang++-3.8 check is because on some Linux distros
clang++-3.8 doesn't exists, while the former does.

r? @bholley

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