Update feature detection to use target_has_atomic

[lscorcia]

Rust 1.60 stabilized the target_has_atomic cfg, adopt it to use the shim on all platforms that require it (e.g. armv5te and more). Fixes #4.

[bltavares]

Hello there, thanks for the PR. There are still a few features to be considered as part of the cfg shim.

I'm not having personal time to dedicate to it at this moment to take a look at this right now.
If you would like, try adding the cfg() for the mutex feature, so other crates can enable the shimmer even on supported platforms if they need to validate it without cross-compiling.

The rust 1.60 should not be needed as well, as it's a shim we should be able to support older versions of rust.

[lscorcia]

Sorry, I don't have the required rust knowledge to complete this work. The issue I had originally was with the indicatif crate, which has now been updated to use https://github.com/taiki-e/portable-atomic . Feel free to close the PR if you feel so, I already closed the issue.