[atomics.syn] Simplify synopsis using "mostly freestanding" (#8411)

Mark the `<atomic>` header `// mostly freestanding`, remove all the
`// freestanding` comments, and add `// hosted` comments for
`atomic_signed_lock_free` and `atomic_unsigned_lock_free`.

Fixes NB US 191-307 (C++26 CD).

Author: dalg24