This patch adds a new -fgo-embedcfg option.  This will be used by the
go command to implement the go:embed directive that is new in the
upcoming Go 1.16 release.  The option doesn't yet do anything, this is
just framework.  Bootstrapped and ran Go testsuite on
x86_64-pc-linux-gnu.  Committed to mainline.

Ian

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