Map the PSP-2000 extra RAM for homebrew PBPs, outside the user partition

Homebrew written for a PSP-2000+ under custom firmware can use the top
32MB of RAM directly without setting MEMSIZE, which leaves the user
partition at its normal size. NJEMU's slim builds do this (#8925).
Previously we didn't map that memory at all for PBPs; MEMSIZE=1 isn't a
workaround either, since it grows the partition and the heap and stacks
land where the program writes.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

Core/MemMap.cpp

@@ -84,6 +84,7 @@ static u8 *m_pUncachedVRAM[4];
 u32 g_MemorySize;
 // Used to store the PSP model on game startup.
 u32 g_PSPModel;
+u32 g_UserPartitionSize;
 
 static MemMapSetupFlags g_setupFlags;