[std_detect] Atomic assumptions

[MabezDev]

This crate assumes that all targets building std, and therefore this crate support atomic operations because of the usage in the feature cache: https://github.com/rust-lang/stdarch/blob/master/crates/std_detect/src/detect/cache.rs#L8. The target this was spotted on was riscv32-imc (esp32c3) espressif/rust-esp32-example#3.

[Amanieu]

The entire std crate assumes that AtomicUsize exists and supports all types of atomic operations (compare_exchange). If your target does not support atomic operations then you cannot use std and must use core and alloc instead.

[MabezDev]

Thanks for the info. It might be able to possible to emulate atomic's in this instance so I'll look into that.