Raise an illegal instruction exception for ecall/scall instructions.

Those instructions are no yet supported by the (exception) controller and have
been ignored until now. By causing an exception, they can at least be handled
in software if needed in the future.

decoder.sv

@@ -513,6 +513,7 @@ module riscv_decoder
             begin
               // environment (system) call
               // TODO: Handle in controller
+              illegal_insn_o = 1'b1;
             end
 
             32'h00_10_00_73:  // ebreak