compiler: move Backend/Linemap creation out of front end.

compiler: move Backend/Linemap creation out of front end.

    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