Disable CFI for weakly linked syscalls
Currently, when enabling CFI via -Zsanitizer=cfi and executing e.g. std::sys::random::getrandom, we can observe a CFI violation. This is the case for all consumers of the std::sys::pal::weak::syscall macro, as it is defining weak functions which don't show up in LLVM IR metadata. CFI fails for all these functions.
Similar to other such cases in
https://github.com/rust-lang/rust/issues/115199, this change stops emitting the CFI typecheck for consumers of the macro via the `#[no_sanitize(cfi)]` attribute.
r? ``````@rcvalle``````
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