Check x86_64 size assertions on aarch64, too

This makes it easier for contributors on aarch64 workstations (e.g. Macs) to
notice when these assertions have been violated.

compiler/rustc_index/src/bit_set.rs

@@ -400,7 +400,7 @@ enum Chunk {
 }
 
 // This type is used a lot. Make sure it doesn't unintentionally get bigger.
-#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+#[cfg(all(any(target_arch = "x86_64", target_arch = "aarch64"), target_pointer_width = "64"))]
 crate::static_assert_size!(Chunk, 16);
 
 impl<T> ChunkedBitSet<T> {