This adds the x86_64 arch and asm directories and a Documentation/x86_64.

It took a bit longer because I first had to make preemption and thread_info
work and also found some other bugs while doing this. The port has been
tested for a long time on UP.

I'm not sure what I should describe.  A lot is based on i386 with
a lot of cleanups. I wrote a paper about it for last year's OLS that describes
most of the changes (ftp://ftp.firstfloor.org/pub/ak/x86_64.ps.gz). It is
a bit outdated now, but should give a good overview.

It currently has a completely cut'n'pasted from others+hacked 32bit
emulation. I hope to clean that up in the future by merging the generic
core of this with other 64bit archs.

Thanks,
-Andi