[Ada] Do not use maximal values from System in runtime units

[Pierre-Marie de Rodat]

This changes a few runtime units to use explicit bounds based on
Long_Long_Integer instead of maximal values from System, which may
become very large when 128-bit types are supported.

They are used for index types and memory byte counts and, therefore,
the magnitude provided by Long_Long_Integer is sufficient for them.

No functional changes.

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

gcc/ada/

	* libgnat/a-cfinve.adb (Int): Use subtype of Long_Long_Integer.
	* libgnat/a-cofove.adb (Int): Likewise.
	* libgnat/a-cgcaso.adb (T): Likewise.
	* libgnat/a-cogeso.adb (T): Likewise.
	* libgnat/g-debpoo.ads (Byte_Count): Use Long_Long_Integer'Size.
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