Cygwin: revert use of CancelSyncronousIo on wait_thread.

It appears this is causing hangs on native x86_64 in similar scenarios
as the hangs on ARM64, because `CancelSynchronousIo` is returning `TRUE`
but not canceling the `ReadFile` call as expected.

Addresses: msys2/MSYS2-packages#4340 (comment)
Fixes: b091b47 ("cygthread: suspend thread before terminating.")
Signed-off-by: Jeremy Drake <cygwin@jdrake.com>

winsup/cygwin/pinfo.cc

@@ -1210,17 +1210,13 @@ proc_waiter (void *arg)
 
   for (;;)
     {
-      DWORD nb, err;
+      DWORD nb;
       char buf = '\0';
 
       if (!ReadFile (vchild.rd_proc_pipe, &buf, 1, &nb, NULL)
-	  && (err = GetLastError ()) != ERROR_BROKEN_PIPE)
+	  && GetLastError () != ERROR_BROKEN_PIPE)
 	{
-	  /* ERROR_OPERATION_ABORTED is expected due to the possibility that
-	     CancelSynchronousIo interruped the ReadFile call, so don't output
-	     that error */
-	  if (err != ERROR_OPERATION_ABORTED)
-	    system_printf ("error on read of child wait pipe %p, %E", vchild.rd_proc_pipe);
+	  system_printf ("error on read of child wait pipe %p, %E", vchild.rd_proc_pipe);
 	  break;
 	}
 

winsup/cygwin/sigproc.cc

@@ -416,11 +416,7 @@ proc_terminate ()
 	     to 1 iff it is a Cygwin process.  */
 	  if (!have_execed || !have_execed_cygwin)
 	    chld_procs[i]->ppid = 1;
-	  /* Attempt to exit the wait_thread cleanly via CancelSynchronousIo
-	     before falling back to the (explicitly dangerous) cross-thread
-	     termination */
-	  if (chld_procs[i].wait_thread
-	      && !CancelSynchronousIo (chld_procs[i].wait_thread->thread_handle ()))
+	  if (chld_procs[i].wait_thread)
 	    chld_procs[i].wait_thread->terminate_thread ();
 	  /* Release memory associated with this process unless it is 'myself'.
 	     'myself' is only in the chld_procs table when we've execed.  We
@@ -1212,11 +1208,7 @@ remove_proc (int ci)
 {
   if (have_execed)
     {
-      /* Attempt to exit the wait_thread cleanly via CancelSynchronousIo
-	 before falling back to the (explicitly dangerous) cross-thread
-	 termination */
-      if (_my_tls._ctinfo != chld_procs[ci].wait_thread
-	  && !CancelSynchronousIo (chld_procs[ci].wait_thread->thread_handle ()))
+      if (_my_tls._ctinfo != chld_procs[ci].wait_thread)
 	chld_procs[ci].wait_thread->terminate_thread ();
     }
   else if (chld_procs[ci] && chld_procs[ci]->exists ())