Add ucontext_t for aarch64-unknown-linux-musl

Author: ifreund

src/unix/linux_like/linux/musl/b64/aarch64/align.rs

@@ -5,3 +5,17 @@ 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,
+    }
+
+    pub struct mcontext_t {
+        __regs: [u64; 36 + 512],
+    }
+}