rust: sync: atomic: Provide stub for `rusttest` 32-bit hosts

For arm32, on a x86_64 builder, running the `rusttest` target yields:

error[E0080]: evaluation of constant value failed
--> rust/kernel/static_assert.rs:37:23
|
37 | const _: () = ::core::assert!($condition $(,$arg)?);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'assertion failed: size_of::<isize>() == size_of::<isize_atomic_repr>()', rust/kernel/sync/atomic/predefine.rs:68:1
|
::: rust/kernel/sync/atomic/predefine.rs:68:1
|
68 | static_assert!(size_of::<isize>() == size_of::<isize_atomic_repr>());
| -------------------------------------------------------------------- in this macro invocation
|
= note: this error originates in the macro `::core::assert` which comes from the expansion of the macro `static_assert` (in Nightly builds, run with -Z macro-backtrace for more info)

The reason is that `rusttest` runs on the host, so for e.g. a x86_64
builder `isize` is 64 bits but it is not a `CONFIG_64BIT` build.

Fix it by providing a stub for `rusttest` as usual.

Fixes: 84c6d36bcaf9 ("rust: sync: atomic: Add Atomic<{usize,isize}>")
Cc: [e-mail]
Reviewed-by: Onur Özkan <[e-mail]>
Acked-by: Boqun Feng <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Miguel Ojeda <[e-mail]>

rust/kernel/sync/atomic/predefine.rs

···
 // as `isize` and `usize`, and `isize` and `usize` are always bi-directional transmutable to
 // `isize_atomic_repr`, which also always implements `AtomicImpl`.
 #[allow(non_camel_case_types)]
+#[cfg(not(testlib))]
 #[cfg(not(CONFIG_64BIT))]
 type isize_atomic_repr = i32;
 #[allow(non_camel_case_types)]
+#[cfg(not(testlib))]
 #[cfg(CONFIG_64BIT)]
+type isize_atomic_repr = i64;
+
+#[allow(non_camel_case_types)]
+#[cfg(testlib)]
+#[cfg(target_pointer_width = "32")]
+type isize_atomic_repr = i32;
+#[allow(non_camel_case_types)]
+#[cfg(testlib)]
+#[cfg(target_pointer_width = "64")]
 type isize_atomic_repr = i64;
 
 // Ensure size and alignment requirements are checked.