Fix 80-bit long double hex formatting

[mborland]

Closes: #154

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (51fc3f4) 88.62% compared to head (547b0ad) 88.64%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #159      +/-   ##
===========================================
+ Coverage    88.62%   88.64%   +0.01%     
===========================================
  Files           63       64       +1     
  Lines         8141     8162      +21     
===========================================
+ Hits          7215     7235      +20     
- Misses         926      927       +1     

Files	Coverage Δ
test/github_issue_154.cpp	100.00% <100.00%> (ø)
test/to_chars_float_STL_comp.cpp	100.00% <100.00%> (ø)
src/to_chars_float_impl.hpp	94.82% <94.11%> (-0.30%)	⬇️

... and 1 file with indirect coverage changes

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