[X86] Remove FeatureRTM from Skylake processor list

Summary:
There are a LOT of Skylakes and later without TSX-NI. Examples:
- SKL: https://ark.intel.com/products/136863/Intel-Core-i3-8121U-Processor-4M-Cache-up-to-3-20-GHz-
- KBL: https://ark.intel.com/products/97540/Intel-Core-i7-7560U-Processor-4M-Cache-up-to-3-80-GHz-
- KBL-R: https://ark.intel.com/products/149091/Intel-Core-i7-8565U-Processor-8M-Cache-up-to-4-60-GHz-
- CNL: https://ark.intel.com/products/136863/Intel-Core-i3-8121U-Processor-4M-Cache-up-to-3_20-GHz

This feature seems to be present only on high-end desktop and server
chips (I can't find any SKX without). This commit leaves it disabled
for all processors, but can be re-enabled for specific builds with
-mrtm.

Matches https://reviews.llvm.org/D53041

Patch by Thiago Macieira

Reviewers: erichkeane, craig.topper

Reviewed By: craig.topper

Subscribers: lebedev.ri, cfe-commits

Differential Revision: https://reviews.llvm.org/D53042

llvm-svn: 344117