[SystemZ] Properly support 16 byte atomic int/fp types and ops. (#73134)

- Clang FE now has MaxAtomicPromoteWidth / MaxAtomicInlineWidth set to 128, and now produces IR
  instead of calls to __atomic instrinsics for 16 bytes as well.
- Atomic __int128 (and long double) variables are now aligned to 16 bytes by default (like gcc 14).
- AtomicExpand pass now expands 16 byte operations as well.
- tests for __atomic builtins for all integer widths, and __atomic_is_lock_free with friends.
- TODO: AtomicExpand pass handles with this patch expansion of i128 atomicrmw:s. As a next step
  smaller integer types should also be possible to handle this way instead of by the backend.