Fix neg pseudo-op to use rs2 instead of rs1

[HasKeyboard]

Currently, the neg pseudo-op in rv_i hardcodes rs2 to zero, which expands to sub rd, rs1, x0 (computing rd = rs1 - 0).

This PR fixes the hardware mapping by accepting rs2 from the user and hardcoding rs1 to zero. This correctly expands to sub rd, x0, rs2 (computing rd = 0 - rs2).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.78%. Comparing base (4644ba3) to head (639fa1a).
⚠️ Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #428      +/-   ##
==========================================
- Coverage   97.08%   96.78%   -0.30%     
==========================================
  Files          14       14              
  Lines         926      933       +7     
==========================================
+ Hits          899      903       +4     
- Misses         27       30       +3     

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