Fix hang on undefined symbol in rmb under pragma undefextern Problem in the pass4.c implementation led to an infinite loop if any instructions failed to resolve. Instead, if a pass through the loop fails to resolve anything, bail out since it's clearly pointless to continue trying.

The lwbasic directory is an in-progress attempt to write a compiler for a
dialect of basic specifically targetted at the 6809 CPU. It is still
incomplete but is included in this distribution in case someone finds the
code useful. It is not built by default.