backend: (riscv) print list of registers to regalloc stats

[superlopuh]

This makes it clearer when reading the assembly later what the allocated registers were, I forgot that zero was counted in the registers, and misinterpreted the counts in the paper.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.90%. Comparing base (ca76864) to head (902bfbc).
Report is 3 commits behind head on main.

Additional details and impacted files

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##             main    #3111   +/-   ##
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  Coverage   89.90%   89.90%           
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  Files         421      421           
  Lines       53264    53264           
  Branches     8257     8257           
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  Hits        47885    47885           
  Misses       4041     4041           
  Partials     1338     1338           

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