rustc_codegen_ssa: cleanup `AtomicOrdering` * Remove unused `NotAtomic` ordering. * Rename `Monotonic` to `Relaxed` - a Rust specific name. * Derive copy and clone.
rustc_codegen_ssa: cleanup `AtomicOrdering` * Remove unused `NotAtomic` ordering. * Rename `Monotonic` to `Relaxed` - a Rust specific name. * Derive copy and clone.