Forbid use of extern "C-unwind" inside standard library

Those libraries are build with `-C panic=unwind` and is expected to
be linkable to `-C panic=abort` library. To ensure unsoundness
compiler needs to prevent a `C-unwind` call to exist, as doing so may leak
foreign exceptions into `-C panic=abort`.

library/proc_macro/src/lib.rs

@@ -36,6 +36,7 @@
 #![feature(strict_provenance)]
 #![recursion_limit = "256"]
 #![allow(internal_features)]
+#![deny(ffi_unwind_calls)]
 
 #[unstable(feature = "proc_macro_internals", issue = "27812")]
 #[doc(hidden)]