posix_spawn: Don't attempt to dup2 identical fds

macOS Big Sur does not allow the user to `posix_spawn_file_actions_adddup2` an
fd to itself. Instead of behaving as a no-op as specified by POSIX
2017.1 (and done by every other operating system), it instead fails with
`EBADFD` in `posix_spawn`. This caused a single tricky-to-identify
failure in GHC's testsuite.

cbits/posix/posix_spawn.c

@@ -42,10 +42,12 @@ setup_std_handle_spawn (int fd,
         return 0;
 
     case STD_HANDLE_USE_FD:
-        if (posix_spawn_file_actions_adddup2(fa, hdl->use_fd, fd) != 0) {
-            *failed_doing = "posix_spawn_file_actions_adddup2";
-            return -1;
-        }
+        if (hdl->use_fd != fd) {
+            if (posix_spawn_file_actions_adddup2(fa, hdl->use_fd, fd) != 0) {
+                *failed_doing = "posix_spawn_file_actions_adddup2";
+                return -1;
+            }
+       }
         return 0;
 
     case STD_HANDLE_USE_PIPE: