Overhaul Xbox config to fully utilize the libc

[thrimbor]

I went through all feature #defines I could find and set them properly (+ minor cleanup of surrounding stuff). This means that SDL now utilizes our libc better, which not only reduces executable size a bit, but also removes the need for our SDL-samples to provide a _exit() function (XboxDev/nxdk#97).

[dracc]

NevolutionX shrunk by a couple of hundred KiB. Works at least as well in XQEMU as before. :+1:

[mborgerson]

Thanks