Many of the major music players have the ability to export playlists of some kind, and many tagging programs like easytag, audiotagtool, and kid3 also have playlist creation ability.
Here's a dedicated playlist generator that came up when I did a quick package search, but it's not gui.
http://royale.zerezo.com/fapg/
Probably one of your biggest considerations though is to be careful about the file paths it uses. I'm guessing that you'll want to use relative paths of some sort, while many tools will export absolute paths, or at least the paths will be based on the file/directory structure of the local tree, and not necessarily relate to the structure that exists on the mobile.
Really though, there's nothing special about an m3u playlist. It's just a simple text file with one file path per line, and a bit of optional metadata. You can just as easily pipe your filelist directly into a text file, insert an "#EXTM3U" line at the top (or not, even that's usually unneeded), and you're done.
