aco: create hw_init_scratch() function for p_init_scratch lowering

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21780>

src/amd/compiler/aco_builder_h.py

@@ -639,6 +639,8 @@ formats = [(f if len(f) == 5 else f + ('',)) for f in formats]
 % endfor
 };
 
+void hw_init_scratch(Builder& bld, Definition def, Operand scratch_addr, Operand scratch_offset);
+
 } // namespace aco
 
 #endif /* _ACO_BUILDER_ */"""