In the stdio cleanup push and pop wrappers, always call libc stubs for
__pthread_cleanup_push/pop_imp instead of symbols also exported from
libthr.

This prevents calls into libthr if libthr is not yet initialized. The
situation occurs e.g. when an LD_PRELOADed object is not linked
against libthr, but the main binary is.

Reported and tested by: jbeich
PR: 220381
Discussed with: vangyzen
Sponsored by: The FreeBSD Foundation
MFC after: 13 days