[PECOFF] Infer subsystem from the entry point function.

If /subsystem option is not specified, the linker needs to infer it from the
entry point function. If "main" or "wmain" is defined, it's a console
application. If "WinMain" or "wWinMain" is defined, it's a GUI application.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@195592 91177308-0d34-0410-b5e6-96231b3b80d8