[Ada] Enable Ada 2020 Put_Image and Image support for tagged types

[Pierre-Marie de Rodat]

Enable support for Ada 2020's Put_Image and Image attributes for
tagged types.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* exp_put_image.adb: Eliminate references to
	Debug_Flag_Underscore_Z.  Change the meaning of the function
	Enable_Put_Image. Previously, a result of False for a tagged
	type would mean that the type does not get a Put_Image (PI)
	routine at all. Now, it means that the type gets a PI routine
	with very abbreviated functionality (just a call to
	Unknown_Put_Image). This resolves problems in mixing code
	compiled with and without the -gnat2022 switch.
	* exp_ch3.adb: Enable_Put_Image no longer participates in
	determining whether a tagged type gets a Put_Image procedure.  A
	tagged type does not get a Put_Image procedure if the type
	Root_Buffer_Type is unavailable. This is needed to support cross
	targets where tagged types are supported but the type
	Root_Buffer_Type is not available.
	* exp_dist.adb: Add workarounds for some problems that arise
	when using the (obsolete?) Garlic implementation of the
	distributed systems annex with Ada 2022 constructs.
	* libgnat/a-sttebu.ads: Workaround a bootstrapping problem.
	Older compilers do not support raise expressions, so revise the
	the Pre'Class condition to meet this requirement without
	changing the condition's behavior at run time.
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