[PATCH v3] rust: make mutually exclusive with CFI_CLANG

[Matthew Maurer]

>
> Cc'ing Matthew & Ramon as well so that they are aware and in case they
> want to comment.
>
> Cheers,
> Miguel

This patch is fine by me - the last patch needed for KCFI to be
functional in Rust just landed upstream last night, so we should
revisit this (in the form of enabling it) once we move to
`rustc-1.79.0` or later. In case anyone wants it for local
experimentation, I have a backport branch [1] which applies these to
the 1.73.0 compiler and enables it in the kernel [2] (not upstreamed
because the feature isn't yet in kernel's `rustc`), which Android will
be using for the Rust binder driver. This patch will require a recent
(last year or so) clang, as it relies on
`-fsanitize-cfi-icall-experimental-normalize-integers`.

[1]: https://github.com/maurer/rust/tree/1.73.0%2Bcfi
[2]: https://android-review.git.corp.google.com/c/kernel/common/+/2930616