PERL_GLOBAL_STRUCT threads issue

A test in op/threads.t roughly equivalent to this:

    use threads;
    sub f {};
    async \&f;

was randomly failing smokes under the combination of PERL_GLOBAL_STRUCT and
ASan.

This is due to the fact that, under PERL_GLOBAL_STRUCT, "global" vars
are actually allocated, and are freed as the very last thing before
exiting (later than perl_destruct()). In the code above, the created
thread is still exeecuting at this point, and so may access those "global"
vars.  ASan can detect that such a var is being accessed after being
freed; in this case due to PL_ppaddr being indexed in call_sv().

This is easily fixed using the PL_veto_cleanup mechanism; don't do those
final frees if there are still threads running.