define and use syscall_XX registers

rather than use hardcoded r0 etc use syscall_X
change the syscalls and interpreter to use them
later use platform to map from syscall_X to actually used register (like r0 in arm)

test/support/risc_interpreter.rb

@@ -49,7 +49,7 @@ module Risc
 
     def get_return
       assert_equal Parfait::Message , @interpreter.get_register(:saved_message).class
-      @interpreter.get_register(:message)
+      @interpreter.get_register(:syscall_1)
     end
 
     # do as many as given ticks in the main, ie main_at more