[otbn,dv] Sort out address selection in bad_load_store.py
[otbn,dv] Sort out address selection in bad_load_store.py

There was a bug before, where the second branch could pick a tgt_addr
that was much smaller than val + min_offset (if val was very big
compared with model.dmem_size).

Also, force correct alignment most of the time.

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan