Push the calls to create Backend and Linemap object out
    of the front end into the back end, and instead pass pointers to
    these objects in the go_create_gogo_args struct. This allows
    for more flexibility in the interfaces used to create the
    objects.
    
    Reviewed-on: https://go-review.googlesource.com/30698

	* go-gcc.h: New file.
	* go-c.h (struct go_create_gogo_args): Add backend and linemap
	fields.
	* go-lang.c: Include "go-gcc.h".
	(go_langhook_init): Set linemap and backend fields of args.
	* go-gcc.cc: Include "go-gcc.h".
	* go-linemap.cc: Include "go-gcc.h".

From-SVN: r240959