Rollup merge of #140660 - RalfJung:more-order, r=WaffleLapkin

remove 'unordered' atomic intrinsics

As their doc comment already indicates, these operations do not currently have a place in our memory model. The intrinsics were introduced to support a hack in compiler-builtins, but that hack recently got removed (see https://github.com/rust-lang/compiler-builtins/issues/788).

compiler/rustc_codegen_gcc/src/builder.rs

@@ -2454,7 +2454,6 @@ impl ToGccOrdering for AtomicOrdering {
         use MemOrdering::*;
 
         let ordering = match self {
-            AtomicOrdering::Unordered => __ATOMIC_RELAXED,
             AtomicOrdering::Relaxed => __ATOMIC_RELAXED, // TODO(antoyo): check if that's the same.
             AtomicOrdering::Acquire => __ATOMIC_ACQUIRE,
             AtomicOrdering::Release => __ATOMIC_RELEASE,