Deprecate Atomic*.weakCompareAndSet and defer to Atomic*.weakCompareAndSetPlain

The methods on VarHandle provide the following naming scheme:

- for read or write methods plain access is unqualified

- for read-modify-write methods volatile access is unqualified

This means the Atomic.weakCompareAndSet and Atomic.weakCompareAndSetVolatile are inconsistent with that naming.

To reduce the confusion it is proposed to:

1) deprecate Atomic*.weakCompareAndSet; and
2) add Atomic*.weakCompareAndSetPlain