backend: (riscv) use named unallocated registers instead of empty init

[superlopuh]

Two related changes:

• removes the default value of "" for the register name
• replaces previous unallocated register init calls with no args with the dedicated UNALLOCATED constants

This reduces the users of the init API, which I'm slowly refactoring.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.15%. Comparing base (539220f) to head (d5da219).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3972      +/-   ##
==========================================
- Coverage   90.15%   90.15%   -0.01%     
==========================================
  Files         459      459              
  Lines       58299    58290       -9     
  Branches     5690     5690              
==========================================
- Hits        52562    52553       -9     
  Misses       4346     4346              
  Partials     1391     1391              

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[alexarice]

I don't know much about riscv but looks reasonable

[compor]

I'm sorry, after a very brief look how does this fundamentally differ from the change of #3926?
It seems like we go back and forth with the unallocated? Is it too verbose or not after all?