Add Wimp_Extend 15, "read slot size"
Wimp_Extend 15 is equivalent to Wimp_SlotSize,-1,-1, with the exception that the sizes returned in R0-R2 are now measured in pages instead of bytes. This allows code to accurately read the amount of free memory on machines with >2GB of RAM, and provides some future-proofing should we ever wish to support tasks with >2GB of RAM (e.g. via a future super-jumbo wimpslot, or more likely via sparse/PMP wimpslots, where the physical size is larger than the logical size).
Currently there's no need to provide a page-based call to set the slot size (~2GB is fine for the forseeable future), so this call only focuses on reading the sizes.
Requires RiscOS/Sources/Kernel!18 (merged)