[libc++] Simplify features for detecting atomics' support. (#75553)

`non-lockfree-atomics` is very similar to `has-64-bit-atomics`; to
simplify, we can have uniform features for atomic types of
increasing sizes (`has-128-bit-atomics`, `has-256-bit-atomics`, etc.).

`is-lockfree-runtime-function` feature was a workaround for the partial
support for large atomic types on older versions of macOS (see
https://reviews.llvm.org/D91911). While we still support macOS 10.14,
conceptually it's simpler to check for support for all the atomic
functionality inside the `has-*-atomics` features, and the workaround is
no longer worth the maintenance cost.