Force an import of the special symbol __STARTUP__ in the C compiler when

Force an import of the special symbol __STARTUP__ in the C compiler when
main() is encountered. Define this symbol in the startup code. This will
automatically force linking of the startup code which can then reside inside
the standard library as any other object file.

git-svn-id: svn://svn.cc65.org/cc65/trunk@3988 b7a2c559-68d2-44c3-8de9-860c34a00d81