MADT enumeration for x86_64

[Michael Neumann]

Hi,

Last year sephe implemented MADT enumeration for i386. These changes
were missing for x86_64. I ported them to x86_64 and now my HP Compaq
6710b laptop is booting SMP (without APIC_IO) on x86_64. I'd like to
commit the changes if no one objects. Almost all changes are identical
to what sephe made.

http://leaf.dragonflybsd.org/~mneumann/madt_enum.patch
http://leaf.dragonflybsd.org/~mneumann/madt_enum.diff

If you give me go, I'll commit.

Regards,

  Michael