Fix issue with newer PPC64 platforms aliasing long double to __float128

[mborland]

See: boostorg/boost#1035

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.89%. Comparing base (6bd2747) to head (8fc1ee0).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #278   +/-   ##
========================================
  Coverage    94.89%   94.89%           
========================================
  Files           71       71           
  Lines         9123     9123           
========================================
  Hits          8657     8657           
  Misses         466      466           

Files with missing lines	Coverage Δ
src/from_chars.cpp	83.78% <ø> (ø)
src/to_chars.cpp	92.94% <ø> (ø)
src/to_chars_float_impl.hpp	83.39% <ø> (ø)

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