[Ada] Reuse Get_Pragma_Arg to handle pragma argument associations

Code cleanup related to looking at pragma Thread_Local_Storage.
Semantics is unaffected.

gcc/ada/

	* exp_ch3.adb (Build_Init_Statements): Reuse Get_Pragma_Arg.
	* exp_prag.adb (Arg_N): Likewise.
2 files changed