Now that we support several Rust versions, introduce
`CONFIG_RUSTC_VERSION` so that it can be used in Kconfig to enable and
disable configuration options based on the `rustc` version.

The approach taken resembles `pahole`'s -- see commit 613fe169
("kbuild: Add CONFIG_PAHOLE_VERSION"), i.e. a simple version parsing
without trying to identify several kinds of compilers, since so far
there is only one (`rustc`).

However, unlike `pahole`'s, we also print a zero if executing failed for
any reason, rather than checking if the command is found and executable
(which still leaves things like a file that exists and is executable,
but e.g. is built for another platform [1]). An equivalent approach to
the one here was also submitted for `pahole` [2].

Link: https://lore.kernel.org/rust-for-linux/CANiq72=4vX_tJMJLE6e+bg7ZECHkS-AQpm8GBzuK75G1EB7+Nw@mail.gmail.com/ [1]
Link: https://lore.kernel.org/linux-kbuild/20240728125527.690726-1-ojeda@kernel.org/ [2]
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20240902165535.1101978-2-ojeda@kernel.orgSigned-off-by: Miguel Ojeda <ojeda@kernel.org>