htl: Move cleanup stack to variable shared between libc and pthread

If libpthread gets loaded dynamically, the stack needs to already contain the
cleanup handlers of the main thread.

* htl/libc_pthread_init.c (__pthread_cleanup_stack): New per-thread variable.
* htl/Versions (libc): Add __pthread_cleanup_stack as private symbol.
* htl/pt-internal.h (struct __pthread): Remove cancelation_handlers
field.
(__pthread_cleanup_stack): Add variable declaration.
* htl/pt-alloc.c (initialize_pthread): Remove initialization of
cancelation_handlers field.
* htl/pt-cleanup.c (__pthread_get_cleanup_stack): Return the address of
__pthread_cleanup_stack instead of that of the cancelation_handlers
field.
* htl/forward.c: Include <pt-internal.h>.
(dummy_list): Remove variable.
(__pthread_get_cleanup_stack): Return the address of __pthread_cleanup_stack
instead of that of dummy_list.