ch6: import x64 files and corresponding modification of Makefile

hypercall-x86_64.h : xen-4.2.1/extras/mini-os/include/x86/x86_64
bootstrap.x86_64.S : xen-4.2.1/extras/mini-os/arch/x86/x86_64.S
loader-x86_64.lds  : ../chapter2/x86_64.lds
NOTE: not yet compiled.
ch6: working in x86_64

Include correct header file. API of HYPERVISOR_set_callbacks is changed.
Implementation of first_bit() and xchg require changing.
Added difinition of pte_t and __pte(x), maybe these 2 macro should be
defined in x86_mm.h.