sim: rx: switch syscalls to common nltvals

Rather than hand duplicate the syscall table, switch to the common
nltvals framework.  We have to tweak the constant names, but we get
everything else for free.  I made sure the constants have the same
values before & after too :).

sim/common/gennltvals.py

@@ -70,6 +70,7 @@ TARGETS = {
     'msp430',
     'pru',
     'riscv',
+    'rx',
     'sparc',
     'v850',
 }