Enable target_feature on any LLVM 6+

Enable target_feature on any LLVM 6+

In `LLVMRustHasFeature()`, rather than using `MCInfo->getFeatureTable()`
that is specific to Rust's LLVM fork, we can use this in LLVM 6:

    /// Check whether the subtarget features are enabled/disabled as per
    /// the provided string, ignoring all other features.
    bool checkFeatures(StringRef FS) const;

Now rustc using external LLVM can also have `target_feature`.