Treat 'amd64' as 'x86_64' as done for other tests

And add a decorator for llvm.org/pr18200

Patch by John Wolfe.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@201708 91177308-0d34-0410-b5e6-96231b3b80d8