Most important, use a correct signature for the
__pthread_cleanup_push_imp() stub, which was incorrectly generated
with two-args variant. The pthread_cleanup_info pointer was corrupted
in the forwarded call to the real libthr implementation, visible on
PowerPC and possibly ARM. [1]
Found and tested by: Mark Millard <markmi@dsl-only.net> [1]
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
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