[X86] Use X86_MC::ParseX86Triple to add mode features to feature string in...

[X86] Use X86_MC::ParseX86Triple to add mode features to feature string in X86Subtarget::initSubtargetFeatures.

Remove mode flags from constructor and remove calls to
ToggleFeature for the mode bits.

By adding them to the feature string we handle initializing the
mode member variables in X86Subtarget and the feature bits in
MCSubtargetInfo in one shot.