Avoid using `spin` if `OnceCell` is available

[briansmith]

The vast majority of the time we are not building for a no_std environment when targeting x86, x86-64, ARM, or AAarch64. If we're not targeting a no_std environment then we can and should use OnceCell and avoid using spin. We should only use spin as a last resort.

Perhaps we should assume that if there is an OS, then OnceCell is available, and if there isn't an OS then assume it isn't available.

[LegNeato]

How about using the newly (1.60) stabilized target_has_atomic?

#[cfg(all(std, target_has_atomic = "ptr"))]

From this old RFC:

After a quick survey of the LLVM and Clang code, architectures can be classified into 3 categories:

1. The architecture does not support any form of atomics (mainly microcontroller architectures).
2. The architecture supports all atomic operations for integers from i8 to iN (where N is the architecture >word/pointer size).
3. The architecture supports all atomic operations for integers from i8 to i(N*2).

OnceCell appears to use atomics: https://github.com/matklad/once_cell/blob/090caea92f21ecba71b743c1c636f3fe9d14b598/src/imp_pl.rs#L5

and

https://github.com/rust-lang/rfcs/pull/2788/commits/07c3c59a969d32b3b30075122887f3de8e29f00c#diff-fefae16108333b139ec1fa67533a9e0fc44fb05f01a1f92c19f09bb6d5168a23R375