Allow access to invoking the ooRexx garbage collector

In the developer list the following mailing thread illustrates a situation, which could probably be remedied, if the ooRexx C++ APIs would allow for explicitly invoking the garbage collector (in the mailing thread use case it would make sense to run the garbage collector immediately after a terminate() of an ooRexx instance).

https://sourceforge.net/mailarchive/forum.php?thread_name=29399.145.64.134.245.1299664533.squirrel%40webmail.internl.net&forum_name=oorexx-devel