Fix some doc links for intrinsics

This fixes a few intrinsic docs that had a link directly to itself
instead of to the correct function in the `mem` module.

Author: ehuss

library/core/src/intrinsics/mod.rs

@@ -2667,7 +2667,7 @@ pub unsafe fn vtable_align(ptr: *const ()) -> usize;
 /// More specifically, this is the offset in bytes between successive
 /// items of the same type, including alignment padding.
 ///
-/// The stabilized version of this intrinsic is [`size_of`].
+/// The stabilized version of this intrinsic is [`core::mem::size_of`].
 #[rustc_nounwind]
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_intrinsic_const_stable_indirect]
@@ -2681,7 +2681,7 @@ pub const fn size_of<T>() -> usize;
 /// Therefore, implementations must not require the user to uphold
 /// any safety invariants.
 ///
-/// The stabilized version of this intrinsic is [`align_of`].
+/// The stabilized version of this intrinsic is [`core::mem::align_of`].
 #[rustc_nounwind]
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_intrinsic_const_stable_indirect]
@@ -2704,7 +2704,7 @@ pub const fn variant_count<T>() -> usize;
 
 /// The size of the referenced value in bytes.
 ///
-/// The stabilized version of this intrinsic is [`size_of_val`].
+/// The stabilized version of this intrinsic is [`core::mem::size_of_val`].
 ///
 /// # Safety
 ///
@@ -2717,7 +2717,7 @@ pub const unsafe fn size_of_val<T: ?Sized>(ptr: *const T) -> usize;
 
 /// The required alignment of the referenced value.
 ///
-/// The stabilized version of this intrinsic is [`align_of_val`].
+/// The stabilized version of this intrinsic is [`core::mem::align_of_val`].
 ///
 /// # Safety
 ///