[otbn] turn modexp into library
[otbn] turn modexp into library

Allow to assemble full RSA encrypt and decrypt
with primitives from modexp.

- global binding for symbol of modload primitive
- replaces any ecall by ret

Signed-off-by: Felix Miller <43d2a12732a784b305469416c0ba5ce1795e752a@gi-de.com>

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