[X86] Remove 'rtm' feature from KNL.

I'm unsure if KNL has this feature, but the backend never thought it did, only clang did. The predefined-arch-macros test lost the check for __RTM__ on KNL when it was removed Skylake CPUs in r344117.

I think we want to drop it from KNL for consistency with Skylake anyway regardless of how we got here.

llvm-svn: 344978