rust: sort global Rust flags

Sort the global Rust flags so that it is easier to follow along when we
have more, like this patch series does.

Reviewed-by: Trevor Gross <tmgross@umich.edu>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20240904204347.168520-3-ojeda@kernel.org


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>