Default enable the rtm feature only on skylake and later for now because Intel...

Default enable the rtm feature only on skylake and later for now because Intel disabled the feature on some haswell and broadwell processors:

http://www.intel.com/content/dam/www/public/us/en/documents/specification-updates/core-m-processor-family-spec-update.pdf

the -mrtm option will still work normally.

llvm-svn: 298956