[coreboot] x86_64 support needs review

[Patrick Rudolph]

Hi folks,
While working on x86_64 support for the qemu mainboard target I gave it 
a try on real hardware
and I got my machine booting with only 3 additional patches.
The current solution seems to work without problems with the following 
specs:

* All stages are compiled in x86_64
* page tables are static and reside in ROM
* Memory above 4GiB isn't accessible yet

Tested on:
* Intel Nehalem
* Intel Ibexpeak
* EHCI debug enabled

Please test and review:

https://review.coreboot.org/q/topic:%22x86_64_support2%22+(status:open)
https://review.coreboot.org/q/topic:%22x86_64_support_real_hardware%22+(stat...)

Kind Regards,
Patrick