Rollup merge of #132397 - m-ou-se:warn-missing-abi, r=Nadrieril

Make missing_abi lint warn-by-default.

This makes the missing_abi lint warn-by-default, as suggested here: https://github.com/rust-lang/rfcs/pull/3722#issuecomment-2447719047

This needs a lang FCP.

example/mini_core.rs

@@ -689,7 +689,7 @@ fn index(&self, index: usize) -> &Self::Output {
     }
 }
 
-extern {
+extern "C" {
     type VaListImpl;
 }
 

example/mini_core_hello_world.rs

@@ -258,13 +258,13 @@ unsafe fn uninitialized<T>() -> T {
 
     assert_eq!(((|()| 42u8) as fn(()) -> u8)(()), 42);
 
-    extern {
+    extern "C" {
         #[linkage = "weak"]
         static ABC: *const u8;
     }
 
     {
-        extern {
+        extern "C" {
             #[linkage = "weak"]
             static ABC: *const u8;
         }

example/mod_bench.rs

@@ -3,7 +3,7 @@
 #![allow(internal_features)]
 
 #[link(name = "c")]
-extern {}
+extern "C" {}
 
 #[panic_handler]
 fn panic_handler(_: &core::panic::PanicInfo<'_>) -> ! {

example/std_example.rs

@@ -7,7 +7,7 @@
 use std::io::Write;
 use std::ops::Coroutine;
 
-extern {
+extern "C" {
     pub fn printf(format: *const i8, ...) -> i32;
 }