compiler: add -fgo-c-header=FILE to create a C header

compiler: add -fgo-c-header=FILE to create a C header

    The new -fgo-c-header=FILE option will write a C header file defining
    all the struct types and numeric const values in package scope.  This
    will be used when building the Go runtime package (libgo/go/runtime) to
    generate a C header file that may be included by the C code in the C
    runtime package (libgo/runtime).

    This will ensure that the Go code and C code are working with the same
    data structures as we convert the runtime from C to Go to upgrade to the
    current GC runtime, notably the concurrent garbage collector.

    Reviewed-on: https://go-review.googlesource.com/28000

	* lang.opt (fgo-c-header, fgo-compiling-runtime): New options.
	* go-c.h (struct go_create_gogo_args): Define.
	(go_create_gogo): Change declaration to take struct pointer.
	* go-lang.c (go_c_header): New static variable.
	(go_langhook_init): Update call to go_create_gogo.
	* gccgo.texi (Invoking gccgo): Document -fgo-c-header and
	-fgo-compiling-runtime.

From-SVN: r239852