exec.c: remove unnecessary #if NB_MMU_MODES

remove unnecessary #if NB_MMU_MODES by using loop.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Acked-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
