arm `rustdoc` Rust 1.85.0-only build error

[Miguel Ojeda <ojeda@kernel.org>]

Hi Christian, Russell, arm, Fabian,

For Rust 1.85.0, for arm32, for the `rustdoc` target (i.e. all those
combined), I am seeing:

      RUSTDOC   .../1.85.0-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/lib.rs
    error: target feature `fp-armv8` cannot be toggled with `#[target_feature]`: Rust ties `fp-armv8` to `neon`
        --> .../1.85.0-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/../../stdarch/crates/core_arch/src/arm_shared/neon/generated.rs:7538:48
         |
    7538 | #[cfg_attr(target_arch = "arm", target_feature(enable = "fp-armv8,v8"))]
         |                                                ^^^^^^^^^^^^^^^^^^^^^

The issue is [1], was introduced in Rust 1.85.0 and was fixed already in
Rust 1.85.1 [2]:

  Link: https://github.com/rust-lang/rust/issues/137366 [1]
  Link: https://github.com/rust-lang/rust/pull/137632 [2]

It is unfortunate since our minimum is going to be 1.85.0 since that is
what Debian Stable has (even if patches may be on top) -- I generally
test the latest patch versions for each minor, but I noticed this since
I also test the actual minimum, and I am bumping it to 1.85.0.

To be clear, it is likely almost no one actually cares about this, since
nobody complained yet, and this can easily be fixed using the already
released Rust 1.85.1.

By the way, what is Debian's policy on upstream Rust patch versions?

Thanks!

Cheers,
Miguel