Use C++ to check for mobile in IOBuf

Summary:
[Folly] Use C++ v.s. the preprocessor to check for mobile in `IOBuf`.

Using C++ allows the compiler, when compiling for any platform, to see the full code for every platform and to typecheck and report errors.

Reviewed By: aditya7fb

Differential Revision: D23365674

fbshipit-source-id: 9cfe048a3114be3d6ef4522e279e19fb21f50a17

folly/io/IOBuf.cpp

@@ -1041,9 +1041,9 @@ void IOBuf::freeExtBuffer() noexcept {
   SharedInfo::invokeAndDeleteEachObserver(
       observerListHead, [](auto& entry) { entry.afterFreeExtBuffer(); });
 
-#if FOLLY_MOBILE
-  buf_ = nullptr;
-#endif
+  if (kIsMobile) {
+    buf_ = nullptr;
+  }
 }
 
 void IOBuf::allocExtBuffer(