Do not use pragma Provide_Shift_Operators in Atree package

Do not use pragma Provide_Shift_Operators in Atree package

This pragma is relatively recent and may be problematic for the bootstrap.

gcc/ada/
	* atree.ads (Slot): Remove pragma Provide_Shift_Operators.
	(Shift_Left): New intrinsic function.
	(Shift_Right): Likewise.
	* atree.adb (Get_1_Bit_Val): Use Natural instead of Integer.
	(Get_2_Bit_Val): Likewise.
	(Get_4_Bit_Val): Likewise.
	(Get_8_Bit_Val): Likewise.
	(Set_1_Bit_Val): Likewise.
	(Set_2_Bit_Val): Likewise.
	(Set_4_Bit_Val): Likewise.
	(Set_8_Bit_Val): Likewise.