ada: Refer to non-Ada binding limitations in user guide

ada: Refer to non-Ada binding limitations in user guide

The limitation of resetting the FPU mode for non 80-bit
precision was not referenced from "Creating a Stand-alone
Library to be used in a non-Ada context". Reference it the same
way it is already referenced from "Interfacing to C".

gcc/ada/

	* doc/gnat_ugn/the_gnat_compilation_model.rst: Reference "Binding
	with Non-Ada Main Programs" from "Creating a Stand-alone Library
	to be used in a non-Ada context".
	* gnat_ugn.texi: Regenerate.