Deprecate the Alpha, IA64 and Sparc64 support

Those architectures don't exist anymore (Alpha, IA64) or aren't widely
used for embedded systems running Linux. Moreover, no clear Buildroot
maintainer has stepped in to maintain these architectures, so it's
better to not pretend that we support them.

The goal is to mark them as deprecated in 2010.08 and remove them in
2010.11.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>