stabilize (const_)ptr_sub_ptr

Author: bend-n

tests/fail/intrinsics/ptr_offset_from_unsigned_neg.rs

@@ -1,6 +1,4 @@
 //@normalize-stderr-test: "\d+ < \d+" -> "$$ADDR < $$ADDR"
-#![feature(ptr_sub_ptr)]
-
 fn main() {
     let arr = [0u8; 8];
     let ptr1 = arr.as_ptr();

tests/pass/ptr_offset.rs

@@ -1,5 +1,4 @@
 //@compile-flags: -Zmiri-permissive-provenance
-#![feature(ptr_sub_ptr)]
 use std::{mem, ptr};
 
 fn main() {