packaging: setup: use arch limit for shmmax

If not already set, set shmmax to arch limit.

Change-Id: Idbc9b651d2f9e47e58c75b44bd84418990f59d60
Bug-Url: https://bugzilla.redhat.com/1039616
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>