Rollup merge of #134013 - BLANKatGITHUB:intrinsic, r=saethlin

Adds new intrinsic declaration

This pr is for #132735 removes removes `extern "intrinsic"`

I think its the last block of this file and was kind of asking for advice how to handle other files as mentioned in the issue .

library/core/src/intrinsics/mod.rs

@@ -3577,7 +3577,6 @@ pub const fn discriminant_value<T>(_v: &T) -> <T as DiscriminantKind>::Discrimin
     unimplemented!()
 }
 
-extern "rust-intrinsic" {
 /// Rust's "try catch" construct for unwinding. Invokes the function pointer `try_fn` with the
 /// data pointer `data`, and calls `catch_fn` if unwinding occurs while `try_fn` runs.
 ///
@@ -3594,8 +3593,16 @@ extern "rust-intrinsic" {
 ///
 /// For more information, see the compiler's source, as well as the documentation for the stable
 /// version of this intrinsic, `std::panic::catch_unwind`.
+#[rustc_intrinsic]
+#[rustc_intrinsic_must_be_overridden]
 #[rustc_nounwind]
-    pub fn catch_unwind(try_fn: fn(*mut u8), data: *mut u8, catch_fn: fn(*mut u8, *mut u8)) -> i32;
+pub unsafe fn catch_unwind(
+    _try_fn: fn(*mut u8),
+    _data: *mut u8,
+    _catch_fn: fn(*mut u8, *mut u8),
+) -> i32 {
+    unreachable!()
+}
 
 /// Emits a `nontemporal` store, which gives a hint to the CPU that the data should not be held
 /// in cache. Except for performance, this is fully equivalent to `ptr.write(val)`.
@@ -3603,8 +3610,11 @@ extern "rust-intrinsic" {
 /// Not all architectures provide such an operation. For instance, x86 does not: while `MOVNT`
 /// exists, that operation is *not* equivalent to `ptr.write(val)` (`MOVNT` writes can be reordered
 /// in ways that are not allowed for regular writes).
+#[rustc_intrinsic]
+#[rustc_intrinsic_must_be_overridden]
 #[rustc_nounwind]
-    pub fn nontemporal_store<T>(ptr: *mut T, val: T);
+pub unsafe fn nontemporal_store<T>(_ptr: *mut T, _val: T) {
+    unreachable!()
 }
 
 /// See documentation of `<*const T>::offset_from` for details.