Support for x86-64 architecture

[ekdahl]

Port Haiku to the x86-64 architecture. Tasks:

• Add cross-compilation support for Haiku/x86-64 to gcc 4.
• Add ELF64 support to boot loader and kernel.
• Extend the x86 boot loader to be able to start an x86-64 kernel.
• Port the kernel itself, i.e. write the x86-64 specific kernel code (CPU, VM, etc.).
• Port fundamental kernel modules to 64 bit/x86-64.
• Port libroot.so and runtime loader to x86-64.
• Port other userland libraries and applications to x86-64.

[wkornewald]

I've at least renamed it here in Trac. :)