Debugger: Add interrupt vector CpuStateX8664.
Debugger: Implement ArchitectureX8664::CreateStackFrame().

This is for the most part a direct copy of the equivalent x86 function
with adjustments to deal with the difference in address sizes and
registers. This gets architectural (aka unassisted) backtraces working
on x86-64, assuming the code in question has been built with frame pointers
enabled.

Thanks to Alex Smith for his assistance in implementing the
function prologue and syscall iframe detection.