powerpc64: Add the clone3 wrapper

It follows the internal signature:

  extern int clone3 (struct clone_args *__cl_args, size_t __size,
 int (*__func) (void *__arg), void *__arg);

The powerpc64 ABI requires an initial stackframe so the child can
store/restore the TOC.  It is create prior calling clone3 by
adjusting the stack size (since kernel will compute the stack as
stack plus size).

Checked on powerpc64-linux-gnu (power8, kernel 6.0) and
powerpc64le-linux-gnu (power9, kernel 4.18).
Reviewed-by: Paul E. Murphy <murphyp@linux.ibm.com>