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

[Andy Moreton <andrewjmoreton@gmail.com>]

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).