[Ada] bindgen: support additional features on targets suppressing the standard lib

[Ada] bindgen: support additional features on targets suppressing the standard lib

gcc/ada/

	* bindgen.adb (Gen_Adainit): For targets that suppress the
	standard library: set the default stack size global variable if
	a value is provided via the -d switch, and generate a call to
	__gnat_initialize_stack_limit if stack checking using stack
	limits is enabled.