stabilize ptr.is_aligned, move ptr.is_aligned_to to a new feature gate

This is an alternative to #121920

Author: Gankra

tests/pass-dep/shims/posix_memalign.rs

@@ -1,6 +1,6 @@
 //@ignore-target-windows: No libc on Windows
 
-#![feature(pointer_is_aligned)]
+#![feature(pointer_is_aligned_to)]
 #![feature(strict_provenance)]
 
 use core::ptr;