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)

lib/slot_machine/macro/putstring.rb

@@ -3,9 +3,13 @@ module SlotMachine
     def to_risc(compiler)
       builder = compiler.builder(compiler.source)
       integer = builder.prepare_int_return # makes integer_tmp variable as return
+      word = Risc.syscall_reg(1).set_compiler(compiler)
+      length = Risc.syscall_reg(2).set_compiler(compiler)
       builder.build do
-        word = message[:receiver].to_reg.known_type(:Word)
-        integer << word[:char_length]
+        string = message[:receiver].to_reg.known_type(:Word)
+        integer << string[:char_length]
+        word << string     # into right
+        length << integer  # registers
       end
       SlotMachine::Macro.emit_syscall( builder , :putstring )
       compiler