runtime: Free runtime created selectors at exit.

When a selector was created using sel_registerName(), it was not free'd
on exit, as selectors not created using sel_registerName did not need
any freeing. This adds a list of selectors created using
sel_registerName() and frees them on exit.

This is only relevant if objc_exit() is used to free all memory occupied
by the runtime. objc_exit() is only useful for debugging memory leaks,
so there was no real world effect.