[X86] Merge X86TargetInfo::setFeatureEnabled and X86TargetInfo::setFeatureEnabledImpl. NFC

setFeatureEnabled is a virtual function. setFeatureEnabledImpl
was its implementation. This split was to avoid virtual calls
when we need to call setFeatureEnabled in initFeatureMap.

With C++11 we can use 'final' on setFeatureEnabled to enable
the compiler to perform de-virtualization for the initFeatureMap
calls.