Rollup merge of #141507 - RalfJung:atomic-intrinsics, r=bjorn3

atomic_load intrinsic: use const generic parameter for ordering

We have a gazillion intrinsics for the atomics because we encode the ordering into the intrinsic name rather than making it a parameter. This is particularly bad for those operations that take two orderings. Let's fix that!

This PR only converts `load`, to see if there's any feedback that would fundamentally change the strategy we pursue for the const generic intrinsics.

The first two commits are preparation and could be a separate PR if you prefer.

`@BoxyUwU` -- I hope this is a use of const generics that is unlikely to explode? All we need is a const generic of enum type. We could funnel it through an integer if we had to but an enum is obviously nicer...

`@bjorn3` it seems like the cranelift backend entirely ignores the ordering?

compiler/rustc_span/src/symbol.rs

@@ -515,6 +515,7 @@ symbols! {
         async_iterator_poll_next,
         async_trait_bounds,
         atomic,
+        atomic_load,
         atomic_mod,
         atomics,
         att_syntax,