Apple: add mach_task_self back

This was removed in 56d665c ("macOs various updates") because tests
failed, but apparently this still exists. Try adding it back.

src/unix/bsd/apple/mod.rs

@@ -6431,6 +6431,7 @@ extern "C" {
         out_processor_infoCnt: *mut mach_msg_type_number_t,
     ) -> ::kern_return_t;
 
+    pub static mut mach_task_self_: ::mach_port_t;
     pub fn task_for_pid(
         host: ::mach_port_t,
         pid: ::pid_t,
@@ -6547,6 +6548,10 @@ extern "C" {
     ) -> ::c_int;
 }
 
+pub unsafe fn mach_task_self() -> ::mach_port_t {
+    mach_task_self_
+}
+
 cfg_if! {
     if #[cfg(target_os = "macos")] {
         extern "C" {