Add OS_DynamicArea 27+28, for supporting lots of RAM
OS_DynamicArea 27 is the same as OS_DynamicArea 5 ("return free memory"), except the result is measured in pages instead of bytes, allowing it to behave sensibly on machines with many gigabytes of RAM.
Similarly, OS_DynamicArea 28 is the same as OS_DynamicArea 7 (internal DA enumeration call used by TaskManager), except the returned size values are measured in pages instead of bytes. A flags word has also been added to allow for more expansion in the future.
Hdr:OSMem now also contains some more definitions which external code will find useful.
Note that these calls are required for RISC OS to properly support machines with ~4GB of RAM, since the current versions of the calls clamp the return values to just under 2GB.