Track undefined sym in the symtab. Remove hack for missing relocs against undef.

src/IceTargetLoweringX8632.h

Index: src/IceTargetLoweringX8632.h
diff --git a/src/IceTargetLoweringX8632.h b/src/IceTargetLoweringX8632.h
index de0e925342acd3564c47ceb68a05ba6ca9d5ab49..25e33a1e9ce761d18eb3fff41ecd04dde7c5402c 100644
--- a/src/IceTargetLoweringX8632.h
+++ b/src/IceTargetLoweringX8632.h
@@ -153,12 +153,11 @@ protected:
   OperandX8632Mem *FormMemoryOperand(Operand *Ptr, Type Ty);
 
   Variable *makeReg(Type Ty, int32_t RegNum = Variable::NoRegister);
+  // Make a call to an external helper function.
   InstCall *makeHelperCall(const IceString &Name, Variable *Dest,
                            SizeT MaxSrcs) {
-    const bool SuppressMangling = true;
     const bool HasTailCall = false;
-    const RelocOffsetT Offset = 0;
-    Constant *CallTarget = Ctx->getConstantSym(Offset, Name, SuppressMangling);
+    Constant *CallTarget = Ctx->getConstantExternSym(Name);
     InstCall *Call =
         InstCall::create(Func, MaxSrcs, Dest, CallTarget, HasTailCall);
     return Call;