core: Move backend related files to separate directories

[compor]

This PR moves the current RISCV backend files (i.e., currently just register allocation strategies) to a directory under xdsl/backend/riscv/ to improve module organization.

[codecov]

Codecov Report

Patch coverage: 93.33% and project coverage change: -0.01 ⚠️

Comparison is base (833c9a1) 86.90% compared to head (f908a9d) 86.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1104      +/-   ##
==========================================
- Coverage   86.90%   86.90%   -0.01%     
==========================================
  Files         138      139       +1     
  Lines       20600    20597       -3     
  Branches     3117     3117              
==========================================
- Hits        17903    17900       -3     
  Misses       2180     2180              
  Partials      517      517              

Impacted Files	Coverage Δ
xdsl/backend/riscv/register_allocation.py	92.85% <92.85%> (ø)
tests/backend/riscv/test_regalloc.py	100.00% <100.00%> (ø)
xdsl/transforms/riscv_register_allocation.py	100.00% <100.00%> (+5.55%)	⬆️

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[tobiasgrosser]

I think this is a great idea.

[superlopuh]

ty

[compor]

@AntonLydike @nazavode?