nptl: Move stack cache management, __libpthread_freeres into libc

This replaces the FREE_P macro with the __nptl_stack_in_use inline
function.  stack_list_del is renamed to __nptl_stack_list_del,
stack_list_add to __nptl_stack_list_add, __deallocate_stack to
__nptl_deallocate_stack, free_stacks to __nptl_free_stacks.

It is convenient to move __libpthread_freeres into libc at the
same time.  This removes the temporary __default_pthread_attr_freeres
export and restores full freeres coverage for __default_pthread_attr.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>