rust: kbuild: pass `-Zunstable-options` for Rust 1.95.0

Custom target specifications are unstable, but starting with Rust 1.95.0,
`rustc` requires to explicitly pass `-Zunstable-options` to use them [1]:

error: error loading target specification: custom targets are unstable and require `-Zunstable-options`
|
= help: run `rustc --print target-list` for a list of built-in targets

David (Rust compiler team lead), writes:

"We're destabilising custom targets to allow us to move forward with
build-std without accidentally exposing functionality that we'd like
to revisit prior to committing to. I'll start a thread on Zulip to
discuss with the RfL team how we can come up with an alternative
for them."

Thus pass it.

Cc: David Wood <david@davidtw.co>
Cc: Wesley Wiser <wwiser@gmail.com>
Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
Link: https://github.com/rust-lang/rust/pull/151534 [1]
Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260206204535.39431-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>