Split out libplatform into a separate libary

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index a324726cc2bf0a2c1e1cad609385814d4db86194..0f75c1df4d4b6ca35f6f43f3610828f5e104de81 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -4931,20 +4931,12 @@ static void* ExternalValue(i::Object* obj) {
 
 
 void v8::V8::InitializePlatform(Platform* platform) {
-#ifdef V8_USE_DEFAULT_PLATFORM
-  FATAL("Can't override v8::Platform when using default implementation");
-#else
   i::V8::InitializePlatform(platform);
-#endif
 }
 
 
 void v8::V8::ShutdownPlatform() {
-#ifdef V8_USE_DEFAULT_PLATFORM
-  FATAL("Can't override v8::Platform when using default implementation");
-#else
   i::V8::ShutdownPlatform();
-#endif
 }