Arbitrary self types v2: pointers feature gate.

The main `arbitrary_self_types` feature gate will shortly be reused for
a new version of arbitrary self types which we are amending per [this
RFC](https://github.com/rust-lang/rfcs/blob/master/text/3519-arbitrary-self-types-v2.md).
The main amendments are:

* _do_ support `self` types which can't safely implement `Deref`
* do _not_ support generic `self` types
* do _not_ support raw pointers as `self` types.

This PR relates to the last of those bullet points: this strips pointer
support from the current `arbitrary_self_types` feature.
We expect this to cause some amount of breakage for crates using this
unstable feature to allow raw pointer self types. If that's the case, we
want to know about it, and we want crate authors to know of the upcoming
changes.

For now, this can be resolved by adding the new
`arbitrary_self_types_pointers` feature to such crates. If we determine
that use of raw pointers as self types is common, then we may maintain
that as an unstable feature even if we come to stabilize the rest of the
`arbitrary_self_types` support in future. If we don't hear that this PR
is causing breakage, then perhaps we don't need it at all, even behind
an unstable feature gate.

[Tracking issue](rust-lang/rust#44874)

This is [step 4 of the plan outlined here](rust-lang/rust#44874 (comment))

tests/pass/dyn-arbitrary-self.rs

@@ -1,6 +1,6 @@
 //@revisions: stack tree
 //@[tree]compile-flags: -Zmiri-tree-borrows
-#![feature(arbitrary_self_types, unsize, coerce_unsized, dispatch_from_dyn)]
+#![feature(arbitrary_self_types_pointers, unsize, coerce_unsized, dispatch_from_dyn)]
 #![feature(rustc_attrs)]
 
 fn pin_box_dyn() {