Allow customization of the user's eln-cache directory

Allow customization of the user's eln-cache directory

* lisp/startup.el (startup-redirect-eln-cache)
(startup--update-eln-cache): New functions.
(startup--original-eln-load-path): New defvar.
(normal-top-level): Record the original value of
'native-comp-eln-load-path' in 'startup--original-eln-load-path'.
Do not amend 'native-comp-eln-load-path' here, as that could
overwrite user customizations.
(command-line): Amend 'native-comp-eln-load-path' after loading
the early-init file, and then again after loading the user init
file.  (Bug#53891)

* etc/NEWS: Announce 'startup-redirect-eln-cache'.