compiler: mark global variables whose address is taken

compiler: mark global variables whose address is taken

To implement this, change the backend to use flag bits for variables.

Fixes https://gcc.gnu.org/PR100537

	PR go/100537
	* go-gcc.cc (class Gcc_backend): Update methods that create
	variables to take a flags parameter.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/322129