As this test is about executing x86_64 code with "lli", we should only
try to do it if we actually execute on an x86_64 target. So instead of
XFAILing individual architectures that can't execute it, instead change
this into requiring an x86_64 target.
Also generalize the target triple form used; don't assume that the
vendor field is set to "pc" - many mingw toolchains use the vendor field
set to "w64".
724eea7654