arch/tile: provide a definition of MAP_STACK

It's convenient for userspace (in particular, glibc) to find a
definition of MAP_STACK.  We use MAP_GROWSDOWN as an alias since
that's appropriate for the main stack, and since our current
allocation of mmap flags bits is running a bit short otherwise.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>