[Ada] Add support for XDR streaming in the default runtime

[Ada] Add support for XDR streaming in the default runtime

2020-06-18  Arnaud Charlet  <charlet@adacore.com>

gcc/ada/

	* Makefile.rtl: Add s-statxd.o.
	* bindgen.adb (Gen_Adainit): Add support for XDR_Stream.
	* bindusg.adb (Display): Add mention of -xdr.
	* gnatbind.adb: Process -xdr switch.
	* init.c (__gl_xdr_stream): New.
	* opt.ads (XDR_Stream): New.
	* libgnat/s-stratt__xdr.adb: Rename to...
	* libgnat/s-statxd.adb: this and adjust.
	* libgnat/s-statxd.ads: New.
	* libgnat/s-stratt.ads, libgnat/s-stratt.adb: Choose between
	default and XDR implementation at runtime.
	* libgnat/s-ststop.ads: Update comments.
	* doc/gnat_rm/implementation_advice.rst: Update doc on XDR
	streaming.
	* gnat_rm.texi: Regenerate.