Fix: [Emscripten] Use non-XDG directories to simplify local storage (#9207)

[embeddedt]

Motivation / Problem

Savegames and config settings are currently not stored properly on Emscripten.

Description

At some point the Emscripten port has started using the XDG-specified directory ($HOME/.local/share/openttd) to store OpenTTD data, however, the persistent IDBFS filesystem is mounted on $HOME/.openttd.

This PR disables XDG support on Emscripten so everything is kept in one place. Since Emscripten is not a full Unix platform with other applications running, there is really no point in using the XDG directory that I see.

Limitations

None.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker