Byte compilation: handle case where the output file is a mountpoint.

Byte compilation: handle case where the output file is a mountpoint.

See Bug#44631.  While testing for a readonly output directory has
slightly different semantics, in practice they should cover cases
where Emacs is sandboxed and can only write to the destination file,
not its directory.

* lisp/emacs-lisp/bytecomp.el (byte-compile-file): Handle the case
where the output directory is not writable.

* test/lisp/emacs-lisp/bytecomp-tests.el
(bytecomp-tests--not-writable-directory)
(bytecomp-tests--dest-mountpoint): New unit tests.