[Ada] Refactor pragma argument getters

[Pierre-Marie de Rodat]

This refactoring replaces exp_prag.adb's Arg1, Arg2 and Arg3 functions
with a new function Arg_N which enables fetching any pragma arguments.

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

gcc/ada/

	* exp_prag.adb (Arg1, Arg2, Arg3): Removed.
	(Arg_N): New function.
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