[Ada] Provide new function Uintp.UI_To_Unsigned_64

[Pierre-Marie de Rodat]

The function UI_To_Int is sometimes not sufficient, so provide this new
function which can be useful to some GNAT back-ends.

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

gcc/ada/

	* uintp.ads, uintp.adb (UI_To_Unsigned_64): New.
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