[otbn] Strengthen sanity check targets in otbn/util/Makefile
[otbn] Strengthen sanity check targets in otbn/util/Makefile

This now assembles and links each input. Now we're generating some
files (in an "otbn" subdirectory of the top-level build directory), we
also touch stamp files for mypy linting to avoid unnecessary
re-checking.

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

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