According to the Java SE 24 API, CompareAndExchange has the
memory semantics as given by VarHandle.compareAndExchange, which has
the following effects [1]:

> Atomically sets the value of a variable to the newValue with the
> memory semantics of setVolatile if the variable's current value,
> referred to as the witness value, == the expectedValue, as accessed
> with the memory semantics of getVolatile.

Thus, the store-release due to setVolatile only occurs if
the compare is successful. Since CASAL already satisfies these
requirements, the DMB does not need to occur to ensure memory ordering
in case the compare fails and a store-release does not happen.

Therefore, we can remove the DMB from cmpxchgptr when LSE
is enabled. We also rename it to cmpxchgptr_barrier to indicate that
this method provides trailing barrier semantics (via either LSE
CASAL or a DMB).

The unused cmpxchgw is removed.

[1] https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/lang/invoke/VarHandle.html#compareAndExchange(java.lang.Object...)