build: enable bindgen

[rina23q]

This PR enables the bindgen feature of rquickjs crate and generates bindgen scripts during the compilation.

Several remarkable points:

• Clang is required, therefore, meta-clang layer is added.
• Specifying LIBCLANG_PATH is required to search libclang.so and other libclang related shared objects.
• BINDGEN_EXTRA_CLANG_ARGS is defined in rust-bindgen. It supports per-target arguments as BINDGEN_EXTRA_CLANG_ARGS_<TARGET>. However, this <TARGET> must be underscore separated triple.
  (e.g. ✅ armv7_poky_linux_gnueabihf, ❌ arm7-poky-linux-gnueabihf)
• For some reason, build.rs of quickjs is also run for host system as well as the target system. To prevent confusion between two, BINDGEN_EXTRA_CLANG_ARGS needs to be defined per target...
• By default, bindgen cannot find the include directory of the target toolchains. If you don't add the specific include path, bindgen uses the host include path.
• TARGET_SYS from Yocto is NOT equal to RUST_TARGET_SYS from meta-lts-mixins-rust. (arm-poky-linux-gnueabi vs armv7-poky-linux-gnueabihf). Critically, the former indicates soft-float, while the latter does hard-float...

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Successor of #380, #374.