Do the indirect jump via %rax on x64

git-svn-id: http://address-sanitizer.googlecode.com/svn/trunk@1975 3c3640eb-14a6-2bb8-0b94-e9e73afe33f5
Minor polishing of how we call the RTL

git-svn-id: http://address-sanitizer.googlecode.com/svn/trunk@1976 3c3640eb-14a6-2bb8-0b94-e9e73afe33f5