backend: (riscv) RiscvRegisterQueue makes empty queue, add helper

[superlopuh]

As a step towards a generic register queue, I wanted to decouple the init from the ABI default available and reserved registers.

[codecov]

Codecov Report

Attention: Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.04%. Comparing base (b85f63b) to head (d415ce9).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
xdsl/backend/riscv/riscv_register_queue.py	87.50%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4060   +/-   ##
=======================================
  Coverage   89.03%   89.04%           
=======================================
  Files         317      317           
  Lines       43338    43351   +13     
  Branches     5403     5407    +4     
=======================================
+ Hits        38588    38601   +13     
+ Misses       3407     3406    -1     
- Partials     1343     1344    +1     

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[compor]

LGTM, in my mind these live in a separate dialect, but this is a good interim step.