Fix alignment of uc_ucontext fields on arm64 android (rust-lang#3894)

[ change `core` to `::core` and move ucontext_t to s_no_extra_traits for
  backport - Trevor ]
(backport <rust-lang#3894>)
(cherry picked from commit 707d32c)

src/unix/linux_like/android/b64/aarch64/align.rs

@@ -4,17 +4,19 @@ s_no_extra_traits! {
     pub struct max_align_t {
         priv_: [f32; 8]
     }
-}
 
-s! {
+    #[allow(missing_debug_implementations)]
     pub struct ucontext_t {
         pub uc_flags: ::c_ulong,
         pub uc_link: *mut ucontext_t,
         pub uc_stack: ::stack_t,
         pub uc_sigmask: ::sigset_t,
+        pub __pad: [u8; 1024 / 8 - ::core::mem::size_of::<::sigset_t>()],
         pub uc_mcontext: mcontext_t,
     }
+}
 
+s! {
     #[repr(align(16))]
     pub struct mcontext_t {
         pub fault_address: ::c_ulonglong,