In save_savedata, modify for only one fwrite

Created by: Protofall

Currently I fwrite each var individually, but I should change this to put all the variables in one buffer then write the single buffer. This will also help with endianness since I only have to endian-ify one buffer.

The data malloc will need to be changed, I could do this with a ifdef for pc to have a bigger size and Dreamcast uses the current size