[turbofan] Add basic support for calling to (a subset of) C functions.

src/compiler/linkage.h

Index: src/compiler/linkage.h
diff --git a/src/compiler/linkage.h b/src/compiler/linkage.h
index abdef7a4efc47eafaf9f32105600c03a6a40d3f0..0358cd28f1b054509750e1a402bb354fa909c7ee 100644
--- a/src/compiler/linkage.h
+++ b/src/compiler/linkage.h
@@ -102,12 +102,18 @@ class CallDescriptor final : public ZoneObject {
   // Returns the kind of this call.
   Kind kind() const { return kind_; }
 
+  // Returns {true} if this descriptor is a call to a C function.
+  bool IsCFunctionCall() const { return kind_ == kCallAddress; }
+
   // Returns {true} if this descriptor is a call to a JSFunction.
   bool IsJSFunctionCall() const { return kind_ == kCallJSFunction; }
 
   // The number of return values from this call.
   size_t ReturnCount() const { return machine_sig_->return_count(); }
 
+  // The number of C parameters to this call.
+  size_t CParameterCount() const { return machine_sig_->parameter_count(); }
+
   // The number of JavaScript parameters to this call, including the receiver
   // object.
   size_t JSParameterCount() const { return js_param_count_; }