ada: Cleanup repeated code in expansion of stream attributes

ada: Cleanup repeated code in expansion of stream attributes

In expansion of various attributes, in particular for the Input/Output
and Read/Write attributes, we can use constants that are already used
for expansion of many other attributes.

gcc/ada/

	* exp_attr.adb (Expand_N_Attribute_Reference): Use constants
	declared at the beginning of subprogram; tune layout.
	* exp_ch3.adb (Predefined_Primitive_Bodies): Tune layout.