[NTOSKRNL] Define the Mm global variables related to Cc on Mm init.

[NTOSKRNL] Define the Mm global variables related to Cc on Mm init.
This means that MmSystemCacheStart, MmSystemCacheEnd, MmSizeOfSystemCacheInPages
have now a valid value.
System cache is not used atm the moment though. MmMapViewInSystemCache() is to
be implemented, and Cc is to be made aware of this.

CORE-14259