posix_spawn: fix stack setup on ia64 [BZ #21275]

The ia64-specific clone2 call expects the base of the stack mapping and
the stack size as sep arguments, not an initial stack value as on other
stack-grows-down architectures.  Reuse the stack-grows-up macro so we
pass in the right stack base.

Reported-by: Matt Turner <mattst88@gentoo.org>