Permit unwinding through FFI by default

See #58794 for context.

Author: Mark-Simulacrum

src/librustc_mir/build/mod.rs

@@ -591,7 +591,7 @@ fn should_abort_on_panic<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
     // unwind anyway. Don't stop them.
     let attrs = &tcx.get_attrs(fn_def_id);
     match attr::find_unwind_attr(Some(tcx.sess.diagnostic()), attrs) {
-        None => true,
+        None => false, // FIXME(#58794)
         Some(UnwindAttr::Allowed) => false,
         Some(UnwindAttr::Aborts) => true,
     }

src/test/run-pass/abort-on-c-abi.rs

@@ -1,4 +1,5 @@
 #![allow(unused_must_use)]
+#![feature(unwind_attributes)]
 // Since we mark some ABIs as "nounwind" to LLVM, we must make sure that
 // we never unwind through them.
 
@@ -10,6 +11,7 @@ use std::io::prelude::*;
 use std::io;
 use std::process::{Command, Stdio};
 
+#[unwind(aborts)] // FIXME(#58794)
 extern "C" fn panic_in_ffi() {
     panic!("Test");
 }