Handle `derive_coerce_pointee` feature and associated `derive(CoercePointee)`

This is used in multiple places in the kernel, grep for CoercePointee:

../rust-for-linux/rust/kernel/list/arc.rs
163:#[derive(core::marker::CoercePointee)]
164-pub struct ListArc<T, const ID: u64 = 0>

../rust-for-linux/rust/kernel/alloc/kbox.rs
103:#[derive(core::marker::CoercePointee)]
104-pub struct Box<#[pointee] T: ?Sized, A: Allocator>(NonNull<T>, PhantomData<A>);

../rust-for-linux/rust/kernel/sync/arc.rs
131:#[derive(core::marker::CoercePointee)]
132-pub struct Arc<T: ?Sized> {
--
541:#[derive(core::marker::CoercePointee)]
542-pub struct ArcBorrow<'a, T: ?Sized + 'a> {

https://doc.rust-lang.org/nightly/core/marker/derive.CoercePointee.html

This is probably going to be hard to implement and exclusively touches typechecking as far as I can tell.

Start by searching the gccrs source for derive_coerce_pointee and CoercePointee, then read the linked Rust documentation for the expected derive behavior. Compare the implementation requirements with the kernel usages in rust/kernel/list/arc.rs, rust/kernel/alloc/kbox.rs, and rust/kernel/sync/arc.rs. Done means gccrs handles the feature and associated derive consistently with those examples.