Add empty zones for Shared Address Space (RFC 6598)

Approved by:	delphij (mentor)
MFC after:	3 days
Sponsored by:	DK Hostmaster A/S
follow up to r254051

- update powerpc/GENERIC64 as well, suggested by mdf
- update comments so that they make sense after the change, suggested by
  jhb

X-MFC after:	never (change specific to head)