[Ada] Fix wrong access to large bit-packed arrays with reverse SSO

[Ada] Fix wrong access to large bit-packed arrays with reverse SSO

2020-06-11  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_pakd.ads: Add paragraph about scalar storage order.
	* exp_pakd.adb (Install_PAT): Do not set the scalar storage
	order of the PAT here but...
	(Set_PB_Type): ...here instead and...
	(Create_Packed_Array_Impl_Type): ...here as well.
	* rtsfind.ads (RE_Id): Add RE_Rev_Packed_Bytes{1,2,4}.
	(RE_Unit_Table): Likewise.
	* libgnat/s-unstyp.ads (Rev_Packed_Bytes1): New derived type.
	(Rev_Packed_Bytes2): Likewise.
	(Rev_Packed_Bytes4): Likewise.