move declaration of __darwin_mcontext64 etc. into align.rs

Author: bsteinb

src/unix/bsd/apple/b64/aarch64.rs renamed to src/unix/bsd/apple/b64/aarch64/align.rs

@@ -0,0 +1,6 @@
+cfg_if! {
+    if #[cfg(libc_align)] {
+        mod align;
+        pub use self::align::*;
+    }
+}