ada: Make Long_Long_Float'Write deterministic

ada: Make Long_Long_Float'Write deterministic

On some platforms, Long_Long_Float'Size (and Long_Long_Float'Stream_Size)
is 128 but only 80 bits are effectively used. This pack makes it so
'Write in this case write zeros for the padding bits instead of
unspecified values.

gcc/ada/ChangeLog:

	* gen_il-fields.ads (Is_IEEE_Extended_Precision): New flag.
	* gen_il-gen-gen_entities.adb: Likewise.
	* gen_il-internals.adb (Image): Likewise.
	* treepr.adb (Image): Likewise.
	* einfo.ads: Document new flag.
	* cstand.adb (Copy_Float_Type, Register_Float_Type): Use new flag.
	* libgnat/s-stratt.ads (W_80IEEE): New procedure.
	* libgnat/s-stratt.adb (W_80IEEE): Likewise.
	* exp_strm.adb (Get_Primitives): Select new procedure when
	appropriate.
	* rtsfind.ads: Register new runtime procedure.
	* sem_ch3.adb (Build_Derived_Numeric_Type,
	Analyze_Subtype_Declaration): Propagate new flag.