rust: allow `clippy::unwrap_or_default` globally

Starting with rustc 1.88, the `clippy::unwrap_or_default` lint triggers
on `rust/kernel/soc.rs` if `CONFIG_CC_OPTIMIZE_FOR_SIZE=y`:

    warning: use of `unwrap_or` to construct default value
      --> ../rust/kernel/soc.rs:66:10
      |
   66 |         .unwrap_or(core::ptr::null())
      |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `unwrap_or_default()`

This is a clippy bug [1]: the lint decides whether an expression is
equivalent to `Default::default()` by inspecting the optimized MIR of
`<*const T as Default>::default` exported by `core`, so its outcome
depends on the optimization level `core` was built with. Moreover, its
suggestion ignores our MSRV of 1.85 (`Default` for `*const T` is only
stable since Rust 1.88), so we could not apply it anyway.

Disable the lint globally rather than working around this single
occurrence; it can be re-enabled conditionally using `rustc-min-version`
once clippy is fixed.

Link: https://github.com/rust-lang/rust-clippy/issues/17379 [1]
Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
Link: https://patch.msgid.link/20260708-soc_unwrap_or-v2-1-007ed724cc7b@nvidia.com
[ Moved to non-versioned group. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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