Move atexit(3) into crtbegin.c and certbeginS.c such that we can pass the

Move atexit(3) into crtbegin.c and certbeginS.c such that we can pass the
right __dso_handle and have dlopen'ed shared objects run their atexit handlers
when they get unloaded.  This is what Linux does, and several ports depend on
this behaviour (and will crash upon exit without this chang).

Based on an earlier diff from matthew@
Tested by ajacoutot@
ok deraadt@