Fix xen_cpuid() inline asm to not clobber stack's red zone

Port xen-unstable changeset 24344:72f4e4cb7440 to kexec-tools:

  Pushing stuff onto the stack on x86-64 when we do not specify
  -mno-red-zone is unsafe. Since the complicated asm is due to register
  pressure on i386, we simply implement an all-new simpler alternative
  for x86-64.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Simon Horman <horms@verge.net.au>