Put main() in a separate file, so that the language

Put main() in a separate file, so that the language
	front-end can use a different main().

	* main.c: New.
	* toplev.c: (main): Rename as toplev_main.
	* toplev.h: Declare toplev_main.
	* Makefile.in (OBJS): add toplev.o.
	  (BACKEND): remove toplev.o, add main.o.

From-SVN: r40247