rust: support Rust >= 1.91.0 target spec

rust: support Rust >= 1.91.0 target spec

commit 8851e27d2cb947ea8bbbe8e812068f7bf5cbd00b upstream.

Starting with Rust 1.91.0 (expected 2025-10-30), the target spec format
has changed the type of the `target-pointer-width` key from string
to integer [1].

Thus conditionally use one or the other depending on the version.

Cc: Waffle Maybe <waffle.lapkin@gmail.com>
Link: https://github.com/rust-lang/rust/pull/144443
Link: https://lore.kernel.org/r/20250829195525.721664-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>