`scripts/rust_is_available.sh` calls `bindgen` with a special
header in order to check whether the `libclang` version in use
is suitable.

However, the invocation itself may fail if, for instance, `bindgen`
cannot locate `libclang`. This is fine for Kconfig (since the
script will still fail and therefore disable Rust as it should),
but it is pretty confusing for users of the `rustavailable` target
given the error will be unrelated:

    ./scripts/rust_is_available.sh: 21: arithmetic expression: expecting primary: "100000 *  + 100 *  + "
    make: *** [Makefile:1816: rustavailable] Error 2

Instead, run the `bindgen` invocation independently in a previous
step, saving its output and return code. If it fails, then show
the user a proper error message. Otherwise, continue as usual
with the saved output.

Since the previous patch we show a reference to the docs, and
the docs now explain how `bindgen` looks for `libclang`,
thus the error message can leverage the documentation, avoiding
duplication here (and making users aware of the setup guide in
the documentation).

Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/rust-for-linux/CAKwvOdm5JT4wbdQQYuW+RT07rCi6whGBM2iUAyg8A1CmLXG6Nw@mail.gmail.com/


Reported-by: François Valenduc <francoisvalenduc@gmail.com>
Closes: https://github.com/Rust-for-Linux/linux/issues/934


Reported-by: Alexandru Radovici <msg4alex@gmail.com>
Closes: https://github.com/Rust-for-Linux/linux/pull/921


Reported-by: Matthew Leach <dev@mattleach.net>
Closes: https://lore.kernel.org/rust-for-linux/20230507084116.1099067-1-dev@mattleach.net/


Fixes: 78521f33 ("scripts: add `rust_is_available.sh`")
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20230616001631.463536-6-ojeda@kernel.org


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>