kbuild: rust_is_available: fix confusion when a version appears in the path

`bindgen`'s output for `libclang`'s version check contains paths, which
in turn may contain strings that look like version numbers [1][2]:

    .../6.1.0-dev/.../rust_is_available_bindgen_libclang.h:2:9: warning: clang version 11.1.0  [-W#pragma-messages], err: false

which the script will pick up as the version instead of the latter.

It is also the case that versions may appear after the actual version
(e.g. distribution's version text), which was the reason behind `head` [3]:

    .../rust-is-available-bindgen-libclang.h:2:9: warning: clang version 13.0.0 (Fedora 13.0.0-3.fc35) [-W#pragma-messages], err: false

Thus instead ask for a match after the `clang version` string.

Reported-by: Jordan Isaacs <mail@jdisaacs.com>
Closes: https://github.com/Rust-for-Linux/linux/issues/942

 [1]
Reported-by: "Ethan D. Twardy" <ethan.twardy@gmail.com>
Closes: https://lore.kernel.org/rust-for-linux/20230528131802.6390-2-ethan.twardy@gmail.com/

 [2]
Reported-by: Tiago Lam <tiagolam@gmail.com>
Closes: https://github.com/Rust-for-Linux/linux/pull/789

 [3]
Fixes: 78521f33 ("scripts: add `rust_is_available.sh`")
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Ethan Twardy <ethan.twardy@gmail.com>
Tested-by: Ethan Twardy <ethan.twardy@gmail.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20230616001631.463536-8-ojeda@kernel.org


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