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
rust-lang#115199, this change stops
emitting the CFI typecheck for consumers of the macro via the
\#[no_sanitize(cfi)] attribute.

library/std/src/sys/pal/unix/weak.rs

@@ -144,6 +144,9 @@ unsafe fn fetch(name: &str) -> *mut libc::c_void {
 #[cfg(not(any(target_os = "linux", target_os = "android")))]
 pub(crate) macro syscall {
     (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
+        // FIXME: Rust currently omits weak function definitions
+        // and its metadata from LLVM IR.
+        #[no_sanitize(cfi)]
         unsafe fn $name($($arg_name: $t),*) -> $ret {
             weak! { fn $name($($t),*) -> $ret }