ada: Fix error message for Stream_Size

ada: Fix error message for Stream_Size

Before this patch, confirming Stream_Size aspect specifications on
elementary types were incorrectly rejected when the stream size was 128,
and the error messages emitted for Stream_Size aspect errors gave
incorrect possible values.

This patch fixes this. The most significant part of the fix is a new
subprogram in Exp_Strm, Get_Primitives, that makes it possible to
retrieve a precise list of supported stream sizes, but also to select
the right runtime streaming primitives for a given type. Using the
latter, this patch factorizes code that was present in both
Build_Elementary_Input_Call and Build_Elementary_Write_Call.

gcc/ada/ChangeLog:

	* exp_strm.ads (Get_Primitives): New function.
	* exp_strm.adb (Get_Primitives): Likewise.
	(Build_Elementary_Input_Call, Build_Elementary_Write_Call): use
	Get_Primitives.
	(Has_Stream_Standard_Rep): Add formal parameter and rename to...
	(Is_Stream_Standard_Rep): New function.
	* sem_ch13.adb (Analyze_Attribute_Definition_Clause): Fix error
	emission.