mm: add getters to retrieve page usage state

This commit adds the following getter interfaces for
Mini-OS's memory management:

mm_total_pages():
 The number of pages assigned to the
 machine.
mm_reserved_pages():
 The number of pages that are not used
 by Mini-OS's memory management.
mm_free_pages():
 The number of pages that are still free
 and can be used for allocations.
mm_heap_pages():
 The number of pages that are allocated
 to the heap (only available when LIBC is
 enabled).