[SND][SSE2] Use PADDSW128 insn for AddSnd().
[BUILD][General] Replace malloc() to GC_malloc(), using garbage corrector on linking.
[VRAM][SSE2] Prefetch RGB8 palette table on drawing.
[Draw][SDL] Faster palette on 4096 colors / Raster diff mode.
[DRAW][SDL] Use __builtin_prefetch() for palette table.
[VRAM] Use __builtin_prefetch for writing data.
[VRAM][SSE2][256K] Use cache for writeptr.