mem_stress: make memory start address configurable

This make it possible to test memories that doesn't start at
address 0, or just parts of memories.
mem_stress: fix fencing error in last address comparison

The comparison for the end address is done with a < check,
and then the loop counter is increased, making the last
access to be done to MEM_STOP.
This is counter-intuitive, where the more natural approach is
to make the last access at MEM_STOP-4.
mem_stress: allow mem_stress program to be located at address 0

This removes the assumption that the mem_stress program is
located somewhere else than at address 0.
The size of the binary is increased slightly by this, since
addresses between 0x160-0x200 are going to waste now.