[otbn] Fix otbn-as for input files that don't end with a newline
[otbn] Fix otbn-as for input files that don't end with a newline

Obviously, such files are evil and should be banned :-) But we
probably shouldn't explode when reading them.

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

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