When using a build.rs script to dynamically generate bindgen bindings
at build-time, it will run on the host, and by default bindgen will
generate bindings for the host arch/libc instead of the target.
To generate the bindings on cross, we need to use BINDGEN_EXTRA_CLANG_ARGS
to specify the proper sysroot and include path for the target.
These arguments are not used for anything other than bindgen's clang
invocation.
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