[otbn] Strip trailing space when matching an instruction in otbn-as
[otbn] Strip trailing space when matching an instruction in otbn-as

Rather embarrassingly, the code:

  nop /* my comment */

failed before this, because operands_str was " " and we were checking
whether it matched the empty regex.

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

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