[otbn] Add named "enums" for flag indices in assembly

These would have avoided one of the problems that Felix had to fix in PR #4459 (and generally should make the code easier to read). The easiest way to do it is probably for `otbn-as` to generate some `.equ` lines at the top of the code it passes to the risc-v assembler.

Start by tracing the otbn-as path that passes assembly to the RISC-V assembler. Check how the generated assembly begins and determine where named .equ lines for flag indices should be emitted. Done means the generated code exposes readable named constants that avoid numeric flag indices and preserves existing assembly behavior.

Written by the indexing model from the issue text.