Revert r321504 "[X86] Don't accidentally enable PKU on cannon lake and icelake...

Revert r321504 "[X86] Don't accidentally enable PKU on cannon lake and icelake or CLWB on cannonlake."

I based that commit on what was in Intel's public documentation here https://software.intel.com/sites/default/files/managed/c5/15/architecture-instruction-set-extensions-programming-reference.pdf

Which specifically said CLWB wasn't until Icelake.

But I've since cross checked with SDE and it thinks these features exist on CNL and ICL. So now I don't know what to believe.

I've added test coverage of the current behavior as part of the revert so at least now have proof of what we're doing.

llvm-svn: 321547