Fix memory mapping for sure this time on 32-bit

Was able to reproduce #6277 with PSP-2000.  We were underallocating the lowmem too, darn.  Fixes #6277 at least for me.

@xsacha afaict Symbian should be fine now with this code, so I took out the extended VRAM thing.  But it'd be good to test it to be sure.

-[Unknown]