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`.

src/rustllvm/PassWrapper.cpp

@@ -205,17 +205,13 @@ GEN_SUBTARGETS
 
 extern "C" bool LLVMRustHasFeature(LLVMTargetMachineRef TM,
                                    const char *Feature) {
-#if LLVM_RUSTLLVM
+#if LLVM_VERSION_GE(6, 0)
   TargetMachine *Target = unwrap(TM);
   const MCSubtargetInfo *MCInfo = Target->getMCSubtargetInfo();
-  const FeatureBitset &Bits = MCInfo->getFeatureBits();
-  const ArrayRef<SubtargetFeatureKV> FeatTable = MCInfo->getFeatureTable();
-
-  for (auto &FeatureEntry : FeatTable)
-    if (!strcmp(FeatureEntry.Key, Feature))
-      return (Bits & FeatureEntry.Value) == FeatureEntry.Value;
-#endif
+  return MCInfo->checkFeatures(std::string("+") + Feature);
+#else
   return false;
+#endif
 }
 
 enum class LLVMRustCodeModel {