Honor DMAC scratchpad address selectors

[GTTeancum]

Summary

• preserve bit 31 as the DMAC SPR selector for MADR and DMAtag ADDR values
• canonicalize SPR-selected addresses to the 16 KiB scratchpad only when resolving transfer memory
• cover direct GIF MADR transfers and chained NEXT tags targeting SPR

Why

DMAC addresses use bit 31 to select scratchpad memory on non-SPR channels. Masking the DMAtag address to 31 bits discarded that selector, while raw bit-31 MADR values were treated as ordinary guest aliases. Both cases could source data from RDRAM instead of SPR.

Testing

• 427/427 ps2x_tests passed (Release)