New atomic load/store operations for the kernel.

Guarantee no fusing and no tearing, and can optionally impose
ordering relative to other memory operations.

Unordered:
- atomic_load_relaxed
- atomic_store_relaxed

Ordered:
- atomic_load_acquire
- atomic_load_consume
- atomic_store_release

These are intended to match C11 semantics, and can be defined in
terms of the C11 atomic API when ready.

(riastradh)