Fails to build on armv5te

[DrSloth]

Hello.
I am using Rocket which depends on this crate and it fails to compile with the armv5te-unknown-linux-musleabi target, because AtomicU64 is not supported and unconditionally used in value/tag.rs.

The error is:

2 | use std::sync::atomic::{Ordering, AtomicU64};
  |                                   ^^^^^^^^^
  |                                   |
  |                                   no `AtomicU64` in `sync::atomic`
  |                                   help: a similar name exists in the module: `AtomicU8`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0432`.
error: could not compile `figment`

[SergioBenitez]

Okay. I wouldn't be surprised if there's more code in Rocket or some of its dependencies that are targeted to 64-bit processors.

I'm open to a fix, but it is not something I will implement myself. Feel free to take it on. The fix should be backwards compatible and have no performance impact on 64-bit processors with the requisite atomic instructions. I don't mind a performance impact on older processors.