[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

Diff for: llvm/include/llvm/MC/MCSubtargetInfo.h

@@ -226,6 +226,14 @@ class MCSubtargetInfo {
     return Found != ProcDesc.end() && StringRef(Found->Key) == CPU;
   }
 
+  ArrayRef<SubtargetSubTypeKV> getCPUTable() const {
+    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.