Fixes for fixed width 128-bit float with specified precision

[mborland]

Also updates the codecov runner to GCC-13 and includes GNU runs so that we can get coverage of the <stdfloat> paths.

[codecov]

Codecov Report

Attention: Patch coverage is 98.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 94.75%. Comparing base (e875722) to head (29cf8ba).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
...lude/boost/charconv/detail/ryu/ryu_generic_128.hpp	97.14%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #232      +/-   ##
===========================================
+ Coverage    94.63%   94.75%   +0.11%     
===========================================
  Files           67       69       +2     
  Lines         8673     9021     +348     
===========================================
+ Hits          8208     8548     +340     
- Misses         465      473       +8     

Files with missing lines	Coverage Δ
src/to_chars.cpp	92.94% <ø> (-0.95%)	⬇️
test/github_issue_166_float128.cpp	100.00% <100.00%> (ø)
test/to_chars_float_STL_comp.cpp	100.00% <100.00%> (ø)
...lude/boost/charconv/detail/ryu/ryu_generic_128.hpp	92.72% <97.14%> (+4.86%)	⬆️

... and 13 files with indirect coverage changes

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