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

(backport <rust-lang#3894>)
(cherry picked from commit 707d32c)

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

@@ -12,6 +12,7 @@ s! {
         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,
     }