Put mach_error_string and mach_task_self back

Signed-off-by: Daniel A. Steffen <dsteffen@apple.com>

Author: KatashiMonosato

src/shims/linux_stubs.c

@@ -40,11 +40,17 @@ unsigned long _dispatch_runloop_queue_probe(dispatch_queue_t dq) {
 void _dispatch_runloop_queue_xref_dispose() { LINUX_PORT_ERROR();  }
 
 void _dispatch_runloop_queue_dispose() { LINUX_PORT_ERROR();  }
+char* mach_error_string(mach_msg_return_t x) {
+  LINUX_PORT_ERROR();
+}
 void mach_vm_deallocate() { LINUX_PORT_ERROR();  }
 
-mach_port_t pthread_mach_thread_np() {
+mach_port_t pthread_mach_thread_np(void) {
   return (pid_t)syscall(SYS_gettid);
 }
+mach_port_t mach_task_self(void) {
+  return (mach_port_t)pthread_self();
+}
 
 /*
  * Stubbed out static data