[otbn] otbn-as: Set ABI in ELF files
[otbn] otbn-as: Set ABI in ELF files

OTBN is based on RV32I and doesn't provide any form of hard float. Pass
the right `-mabi` argument to the GNU assembler to reflect this fact in
the ELF file.

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

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