Add ucontext_t/mcontext_t for aarch64 Android

This ended up just getting copied from Linux as it appears to be the
same.

Author: alexcrichton

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

@@ -5,3 +5,25 @@ s_no_extra_traits! {
         priv_: [f32; 8]
     }
 }
+
+s! {
+    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 uc_mcontext: mcontext_t,
+    }
+
+    #[repr(align(16))]
+    pub struct mcontext_t {
+        pub fault_address: ::c_ulonglong,
+        pub regs: [::c_ulonglong; 31],
+        pub sp: ::c_ulonglong,
+        pub pc: ::c_ulonglong,
+        pub pstate: ::c_ulonglong,
+        // nested arrays to get the right size/length while being able to
+        // auto-derive traits like Debug
+        __reserved: [[u64; 32]; 16],
+    }
+}