Support 16-bit target platforms (#2401)

Since we currently can't roll our pinned nightly toolchain, and since
support for the avr target which initially caused us to notice this
issue was only added on nightly recently, this commit does not add a
regression test. That will be done later as part of #2400 once we are
able to roll our pinned nightly toolchain.

Makes progress on #2400

gherrit-pr-id: I2ce6e9e0c2ba1cbe24c8cc5686e5b9b519bbf102

Author: joshlf

src/layout.rs

@@ -117,11 +117,19 @@ impl DstLayout {
     ///   The alignment value must be a power of two from 1 up to
     ///   2<sup>29</sup>.
     #[cfg(not(kani))]
+    #[cfg(not(target_pointer_width = "16"))]
     pub(crate) const CURRENT_MAX_ALIGN: NonZeroUsize = match NonZeroUsize::new(1 << 28) {
         Some(max_align) => max_align,
         None => const_unreachable!(),
     };
 
+    #[cfg(not(kani))]
+    #[cfg(target_pointer_width = "16")]
+    pub(crate) const CURRENT_MAX_ALIGN: NonZeroUsize = match NonZeroUsize::new(1 << 15) {
+        Some(max_align) => max_align,
+        None => const_unreachable!(),
+    };
+
     /// Constructs a `DstLayout` for a zero-sized type with `repr_align`
     /// alignment (or 1). If `repr_align` is provided, then it must be a power
     /// of two.

src/util/macro_util.rs

@@ -98,6 +98,7 @@ impl<T, U> MaxAlignsOf<T, U> {
     }
 }
 
+#[cfg(__ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS)]
 const _64K: usize = 1 << 16;
 
 // TODO(#29), TODO(https://github.com/rust-lang/rust/issues/69835): Remove this