um: this_cpu_cmpxchg16b_emu

[Richard Weinberger]

Hi,

This patch implements this_cpu_cmpxchg16b_emu() for UML.
As I'm not an amd64 assembly guru I'm not sure whether the assembly part is
correct.
Can someone please review it?

Especially I'm unsure which register have to be saved before each 
call to C functions.

Thanks,
//richard