back.rtlil: implement remaining format specifiers.

[wanda-phi]

This requires a Yosys version from git. The requirement should be bumped to a proper release before Amaranth 0.5.

[whitequark]

Thanks!

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 90.90%. Comparing base (d3c5b95) to head (2381f3c).

Files	Patch %	Lines
amaranth/back/rtlil.py	90.90%	1 Missing and 1 partial ⚠️
amaranth/back/verilog.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1280      +/-   ##
==========================================
+ Coverage   90.84%   90.90%   +0.06%     
==========================================
  Files          44       44              
  Lines       10571    10588      +17     
  Branches     2577     2583       +6     
==========================================
+ Hits         9603     9625      +22     
+ Misses        777      776       -1     
+ Partials      191      187       -4     

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