[rust] Add accessors for MCSubtargetInfo CPU and Feature tables

This is needed for `-C target-cpu=help` and `-C target-feature=help` in rustc

Author: bitshifter

llvm/include/llvm/MC/MCSubtargetInfo.h

@@ -234,6 +234,11 @@ class MCSubtargetInfo {
     return ProcDesc;
   }
 
+
+  ArrayRef<SubtargetFeatureKV> getFeatureTable() const {
+    return ProcFeatures;
+  }
+
   virtual unsigned getHwMode() const { return 0; }
 
   /// Return the cache size in bytes for the given level of cache.