executor: gate AtomicPtr on target_has_atomic, not platform-avr

[SimonIT]

TaskHeader::executor/all_tasks_next used core::sync::atomic::AtomicPtr unconditionally, falling back to portable_atomic::AtomicPtr only under feature = "platform-avr". AVR isn't the only target with no native pointer-wide atomics (max-atomic-width: 0), so any other such target using this field hit a hard compile error instead of falling back.

Gate both cfg branches on target_has_atomic = "ptr" instead, matching run_queue/state's existing pattern in the same file, and make portable-atomic a plain (non-optional) dependency since it's now needed outside of platform-avr too.

[Dirbaio]

We only use load/store on this AtomicPtr. target_has_atomic is false for targets that support atomic load/store but not CAS. What we want here is target_has_atomic_load_store which is unstable. rust-lang/rust#94039

thumbv6m is one such target that can do atomic load/store but not CAS. Merging this would hurt perf on that one.

[Dirbaio]

what target are you running into this problem with? can we just add it?

also, never enable critical-section on portable-atomic from a lib. There's other ways to polyfill like unsafe-assume-single-core, the choice should always be up to the end user.

[SimonIT]

I'm trying to run Embassy on an MC1322x, which appears to lack any atomics and is pretty old. It works with this fix