pthread_equal(): do not assume pthread_t is a comparable type

Anyway, the return value from pthread_self() being the only legal
pthread_t value in the case of stubs, all threads are one and the same.
In other words, pthread_equal() is always true.

Reviewed-by: Julien Cristau <jcristau@debian.org>
Signed-off-by: Peter Harris <pharris@opentext.com>