Add weak stubs for pthread_cancel API.
Adjust 'pthread_[un]register_cancel undefined ref' workaround (Pthreads)

* misc.c (__pthread_register_cancel, __pthread_unregister_cancel):
Move prototypes to pthread_support.c.
* pthread_support.c (__pthread_unregister_cancel,
__pthread_unregister_cancel): Declare only if
PTHREAD_REGISTER_CANCEL_WEAK_STUBS defined.
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