sim: gennltvals.sh: handle split out newlib source tree

sim: gennltvals.sh: handle split out newlib source tree

Since newlib no longer shares the same repo as binutils/gdb, we have to go
searching further afield to locate the sources.  We still look at the top
level for newlib, but if that is not found, we also try up one dir outside
of this source tree.  It sucks, but better than the status quo (no workie).