`libatomic` is mandatory even when platform does not need it

Nobody has claimed this yet.

Few older platforms require `libatomic` for atomic operations but many of them don't (ARMv7 and higher, i686 and higher). `mimalloc` however uses it irrespective of target platform when `libatomic` is installed making it also part of public interface meaning libraries depending on `mimalloc` also need `libatomic`:
https://github.com/microsoft/mimalloc/blob/f2712f4a8f038a7fb4df2790f4c3b7e3ed9e219b/CMakeLists.txt#L229-L232

Would be good if:
- `mimalloc` checked if `libatomic` is really required for target platform (by trying to compile representative atomic operation used within project)
- even if required I don't see a reason for `libatomic` being part of public interface so probably better to make it private

Start at CMakeLists.txt lines 229-232 and inspect how libatomic is detected and exposed to dependents. Check representative atomic operations on the target platforms, then verify that libatomic is linked only when required and is not part of mimalloc's public interface.

Written by the indexing model from the issue text.