Don't needlessly (re-)create default directories on apple platforms

[warmenhoven]

The custom.ini file does not work on apple platforms because apps get executed with a working directory of /. Also the configured directories will get created as needed if not present.

This same PR probably applies to other platforms as well but without more certainty/testing I didn't want to assume.

[zoltanvb]

Just to have at least some searchable reference of your investigations :)
So, the way to avoid re-creating default directories when retroarch.cfg exists and contains different directories, is to put a file named "custom.ini" to the default RetroArch working directory, which is different per platform. File can be empty, contents are irrelevant.