ANNOUNCE: powerpc64 now in -CURRENT

[Nathan Whitehorn]

As of r210025, 64-bit PowerPC support is now present in -CURRENT.
Thanks to all who tested this branch and gave feedback, in particular
Patrick Kerharo and Andreas Tobler for consultation and endless
testing, and to Peter Grehan for prodding me into doing this.

Supported hardware:
- CPUs: PowerPC 970, Cell, POWER4, POWER5, POWER6, POWER7
- Machines: Apple Power Macintosh G5, Apple iMac G5, IBM Cell
  Simulator, Sony Playstation 3 (coming soon)

New Features:
- Support for > 2 GB of RAM
- ZFS (patches under review, not yet in -CURRENT)
- Support for CPUs not implementing the 64-bit bridge mode, such as
  the Cell, POWER6, and POWER7

Caveats:
- Support for ports on powerpc64 is not as good as on powerpc -- in
  particular, Gecko ports like thunderbird and firefox will not build at
  present. Some other ports require minor changes currently in
  uncommitted PRs:
  http://www.freebsd.org/cgi/query-pr-summary.cgi?text=powerpc64

Upgrade instructions:

The safest way to move from a 32-bit to a 64-bit system is to reinstall
the system from scratch. However, there are not yet install CDs (this
should be fixed soon), and some people may not wish to do this. So, for
the adventurous:

cd /usr/src/sys/boot
make
make install
cd /usr/src
make buildworld buildkernel TARGET=powerpc TARGET_ARCH=powerpc64
make installkernel TARGET=powerpc TARGET_ARCH=powerpc64 DESTDIR=/
[reboot - you should now have a 64-bit kernel with a 32-bit world]

cd /usr/src
export UNAME_p=powerpc
make installworld TARGET=powerpc TARGET_ARCH=powerpc64 DESTDIR=/
[reboot - you should now either have a fully 64-bit or a completely
hosed system]
mergemaster -U
[reboot]

-Nathan