It's a leaky abstraction over the underlying system call mechanism. Because the user stack lives in user space and accessing it from the kernel requires the same dance as any other access to user memory, system call arguments are instead passed in registers where they are immediately available to the kernel with no possibility of faults or TOCTTOU holes.

Here 'unsigned long' is just a convenient stand-in for "generic register-sized argument".

A "void * pointing to struct" requires a copy from user space. If a particular prctl() does need a struct, it can certainly stuff a pointer into one of those 'unsigned long' parameters (in the kernel environment, a pointer can be converted to and from an unsigned long without concern).