Add low-level atomic instructions to spirv_std::arch

[BeastLe9enD]

Hey, I added some atomic instructions to spirv_std::arch.
There was already a discussion to add them, but it wasn't done at the time because the idea was to implement atomic using core::sync::atomic (see #442).
But I think it's ok to add the intrinsics in advance and then build the API on top of them later 🙂

[expenses]

I'm just quicky skimming over this on my phone, but I think these would probably be fine with out the _ext on the end of the function name. Everything else looks good though! 👍

[BeastLe9enD]

Okay, I removed the ext suffix 🙂

[BeastLe9enD]

I saw that an atomics branch already existed, so I took the documentation from there and added the Number trait as well.
Also, in the long run, it would be better to use the Scope and Semantics enum types, but that would require adt_const_params and generic_const_exprs, which are not stable yet, so I decided to stick with u32 for now

[eddyb]

Only noticed one comment nit, but otherwise LGTM.

If @expenses and/or @hrydgard are fine with landing this, feel free to merge or ping me to do so.

[eddyb]

@BeastLe9enD I just tried pushing a commit fixing the test, but either the main branch has different rules, or you opted out of the feature, either way, you need to make this change:

diff --git a/tests/ui/arch/debug_printf_type_checking.stderr b/tests/ui/arch/debug_printf_type_checking.stderr
index f719d0aba..d422023e6 100644
--- a/tests/ui/arch/debug_printf_type_checking.stderr
+++ b/tests/ui/arch/debug_printf_type_checking.stderr
@@ -100,9 +100,9 @@ error[E0277]: the trait bound `{float}: Vector<f32, 2_usize>` is not satisfied
               <IVec3 as Vector<i32, 3_usize>>
             and 9 others
 note: required by a bound in `debug_printf_assert_is_vector`
-   --> $SPIRV_STD_SRC/lib.rs:144:8
+   --> $SPIRV_STD_SRC/lib.rs:145:8
     |
-144 |     V: crate::vector::Vector<TY, SIZE>,
+145 |     V: crate::vector::Vector<TY, SIZE>,
     |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `debug_printf_assert_is_vector`

 error[E0308]: mismatched types

(line numbers in spirv-std changed, the test isn't broken, only differs in diagnostic details)

Or cargo compiletest --bless if that's easier for you (i.e. if it won't require a rebuild etc.).

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Sorry this fell through the cracks, I don't remember hearing from anyone after posting the above review (#877 (review)) (EDIT: oh, @expenses brought it up, but I missed it in the context of #888).