bug#46494: 28.0.50; [native-comp] Problems with async background compile

[Andy Moreton]

Problems notes with async compile in native-comp branch on Windows:

a) Bug #46256 describes problems with AOT compiled native-comp emacs not
   finding prebuilt .eln files when built for mingw64 64bit on Windows.

   As a result, emacs complains with an echo area warning for every .eln
   file that it cannot find in the expected location.

   The stream of frequent warnings that causes make emacs mostly
   unresponsive to user input.

b) The "background" async compilation of .eln files is CPU intensive and
   somewhat slow. The default settings run a compile on every available
   core, which is unfriendly for other workloads running on the same
   machine.

   It would be helpful to users to have a command to show the state of
   the async background compilation, including the running compile
   processes and the queue of pending compilation requests.

c) Quitting emacs when async compilation processes are running sometimes
   causes crashes in the compile processes, which show the emacs abort
   dialog (once for each async process). The dialogs disappear after a
   short delay (presumably due to the parent emacs having exited).