Fix size/align of ucontext_t on aarch64-apple-darwin

This commit effectively reverts rust-lang#2817. Currently `ucontext_t` has both
the wrong size and the wrong alignment for aarch64-apple-darwin which
causes problems for users referencing the structure [1]. The issue
linked from rust-lang#2817 claimed that it fixed rust-lang#2812 but that's still an issue
where FFI warnings are still emitted for usage of `ucontext_t` due to
its transitive usage of `u128`. I'm not sure how to fix rust-lang#2812 myself,
but given that rust-lang#2817 doesn't appear to solve its original intent and
additionally the size/align are currently wrong this commit reverts in
the meantime.

[1]: bytecodealliance/wasmtime#6785 (comment)

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

@@ -15,7 +15,6 @@ s! {
         pub uc_link: *mut ::ucontext_t,
         pub uc_mcsize: usize,
         pub uc_mcontext: mcontext_t,
-        __mcontext_data: __darwin_mcontext64,
     }
 
     pub struct __darwin_mcontext64 {