Introduced -nomain flag to disable generation of main(),

in assumption that it is provided by one of the libraries.

runtime/src/launcher/cpp/main.cpp

@@ -1,5 +0,0 @@
-extern "C" int Konan_main(int argc, char** argv);
-
-int main(int argc, char** argv) {
-  return Konan_main(argc, argv);
-}