module/rust: set _FILE_OFFSET_BITS=64 for bindgen

Meson sets 64-bit offsets as the default for all platforms but MSVC.
Lets do the same for bindgen, to ensure we get compatible definitions.
Do this by calling `get_always_args()` on the first C'ish host compiler
we can find.

Note that the `libc` crate does not expose 64-bit types as the default
and there is no intention to do so. Instead, it exposes 32-bit default
types, plus the 64-bit extended types with the `*64` suffix. This is
quite unfortunate, but it seems unlikely to change [1].

However, use of `bindgen` is usually not tied to the `libc` crate.
Instead, it is tied to whatever other C code in the same project does.
And Meson sets `_FILE_OFFSET_BITS=64` unconditionally for all this C
code. It thus seems much more plausible for Meson to also imply it for
bindgen.

Given that Rust code that uses the `libc` crate very likely already uses
the `*64` suffixed variants, they are unaffected by whether
`_FILE_OFFSET_BITS=64` is set. If they use `libc::off_t`, they already
explicitly state that they use the 32-bit variant on 32-bit platforms.
Hence, it is inherently incompatible to C code that uses
`_FILE_OFFSET_BITS=64`.

And lastly, if a Meson project is Rust-only, but generates its internal
code from its public C headers, then it is better suited to actually
call `add_language('c')` and ensure that Meson knows what the compiler
configuration for the target platform actually is. Otherwise, bindgen
cannot know what platform options to enable. Hence, warn loudly if
`rust.bindgen()` is used without a configured C compiler (even if the
compiler used by bindgen does not necessarily match the configured one).

[1] https://github.com/rust-lang/libc/issues/3223#issuecomment-2033298952

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