[Ada] ACATS 4.1Q - CDD2A04 - RLS not followed

[Ada] ACATS 4.1Q - CDD2A04 - RLS not followed

2020-06-12  Arnaud Charlet  <charlet@adacore.com>

gcc/ada/

	* exp_strm.adb (Build_Elementary_Input_Call): Add support for 24
	bits elementary types.
	* rtsfind.ads: Add 24 bits integer streaming routines.
	* sem_ch13.adb (Analyze_Attribute_Definition_Clause
	[Attribute_Stream_Size]): Add support for 24 bits elementary
	types.
	* libgnat/s-stratt.ads, libgnat/s-stratt.adb,
	libgnat/s-stratt__xdr.adb: Add support for signed and unsigned
	24 bits integers.