PR20672 prime: prevent python $var infinitely recursive expansion

PR20672 prime: prevent python $var infinitely recursive expansion

The new python expanding visitors take probe-python bodies and intend
to generate probe-process bodies, expanding python $var references to
uprobe marker $var ones.  During the expansion process however, the
code can get confused as to whether the new $arg3 is fish or foul.
If asked, it will map $arg3 -> foo($arg3, "arg3"), then recurse,
which is a recipe for stack exhaustion.

So now we map the generated $arg3 with a new hacky target_symbol flag,
"synthetic", to prevent infinite recursion there.  (Note that using
the recently added abort_provide() response from the expander is not
enough, because an outer relaxation loop would then trigger infinite
looping rather than infinite recursion.)

A better solution could be to have a *not-in-place* rewriting visitor,
so that the output tree is never reprocessed by the invoking visitor.