compiler: support -fgo-importcfg

	* lang.opt (fgo-importcfg): New option.
	* go-c.h (struct go_create_gogo_args): Add importcfg field.
	* go-lang.cc (go_importcfg): New static variable.
	(go_langhook_init): Set args.importcfg.
	(go_langhook_handle_option): Handle -fgo-importcfg.
	* gccgo.texi (Invoking gccgo): Document -fgo-importcfg.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/506095