Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Fix build with gcc. Move sf_buf_alloc()/sf_buf_free() declarations
to MD headers.

Add support for 64-bit PowerPC CPUs operating in the 64-bit bridge mode
provided, for example, on the PowerPC 970 (G5), as well as on related CPUs
like the POWER3 and POWER4.

This also adds support for various built-in hardware found on Apple G5
hardware (e.g. the IBM CPC925 northbridge).

Reviewed by: grehan

Rework and extend PowerPC headers definitons towards Book-E/e500 CPUs support.

Approved by: cognet (mentor)
Obtained from: Juniper, Semihalf
MFp4: e500

MFamd64
Simplify the sf_buf implementation. In short, make it a veneer
over the direct virtual-to-physical mapping.

- Modify alpha's sf_buf implementation to use the direct virtual-to-
physical mapping.
- Move the sf_buf API to its own header file; make struct sf_buf's
definition machine dependent. In this commit, we remove an
unnecessary field from struct sf_buf on the alpha, amd64, and ia64.
Ultimately, we may eliminate struct sf_buf on those architecures
except as an opaque pointer that references a vm page.