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

This is an alternative to #121920

compiler/rustc_codegen_gcc/patches/libgccjit12/0001-core-Disable-portable-simd-test.patch

@@ -14,7 +14,7 @@ index d0a119c..76fdece 100644
 @@ -89,7 +89,6 @@
  #![feature(never_type)]
  #![feature(unwrap_infallible)]
- #![feature(pointer_is_aligned)]
+ #![feature(pointer_is_aligned_to)]
 -#![feature(portable_simd)]
  #![feature(ptr_metadata)]
  #![feature(lazy_cell)]
@@ -27,6 +27,6 @@ index d0a119c..76fdece 100644
  mod slice;
  mod str;
  mod str_lossy;
--- 
+--
 2.42.1