updated calling convention comments in em64t_ist.s.
updated calling convention comments in em64t_ist.s.

jlmucb/cloudproxy,tmroeder/cloudproxy,cjpatton/cloudproxy,cjpatton/cloudproxy,tmroeder/cloudproxy,jethrogb/cloudproxy,cjpatton/cloudproxy,William-J-Earl/cloudproxy,jethrogb/cloudproxy,kevinawalsh/cloudproxy,tmroeder/cloudproxy,jlmucb/cloudproxy,jethrogb/cloudproxy,tmroeder/cloudproxy,jethrogb/cloudproxy,cjpatton/cloudproxy,cjpatton/cloudproxy,William-J-Earl/cloudproxy,jlmucb/cloudproxy,jlmucb/cloudproxy,William-J-Earl/cloudproxy,William-J-Earl/cloudproxy,William-J-Earl/cloudproxy,jlmucb/cloudproxy,kevinawalsh/cloudproxy,jethrogb/cloudproxy,tmroeder/cloudproxy