[Ada] Preliminary cleanup in floating-point output implementation

[Ada] Preliminary cleanup in floating-point output implementation

[Pierre-Marie de Rodat]

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This moves the low-level processing on characters to the package
Img_Util, factors out common processing with fixed point, and overhauls
the handling of negative zeros and rounding.  This also removes the
Is_Negative intrinsic function, which was only accessible from
predefined units and is now unused.

No functional changes.

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

gcc/ada/

	* exp_intr.adb: Remove with/use clauses for Urealp.
	(Expand_Is_Negative): Delete.
	(Expand_Intrinsic_Call): Do not call it.
	* rtsfind.ads (RE_Id): Remove RE_Float_Unsigned.
	(RE_Unit_Table): Remove entry for RE_Float_Unsigned.
	* snames.ads-tmpl (Name_Is_Negative): Delete.
	* libgnat/s-imgrea.ads (Set_Image_Real): Fix mode of S parameter.
	* libgnat/s-imgrea.adb: Add with/use clauses for System.Img_Util.
	(LLU): New subtype.
	(Maxdigs): Use it.
	(Is_Negative): Reimplement.
	(Image_Floating_Point): Simplify.
	(Set_Image_Real): Fix mode of S parameter.  Remove the low-level
	processing on characters.  Flip the sign of the Scale variable.
	Compute the maximum number of digits for the straight notation.
	Call Set_Decimal_Digits at the end to do the final formatting.
	* libgnat/s-imguti.ads (Floating_Invalid_Value): New type.
	(Set_Floating_Invalid_Value): New procedure.
	* libgnat/s-imguti.adb (Set_Floating_Invalid_Value): Implement it
	based on existing code from Set_Image_Real.
	* libgnat/s-unstyp.ads (Float_Unsigned): Delete.

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