Rollup merge of #154618 - CAD97:atomicptr-update-cfg-is-not-eight, r=joshtriplett

Fix AtomicPtr::update's cfg gate

I'm *pretty* sure this is supposed to be `#[cfg(target_has_atomic = "ptr")]` like `AtomicPtr::try_update` is.

cc @GrigorenkoPV (author), @Noratrieb (r+ to rust-lang/rust#133829)