Make __float128 a floating point type.

[jzmaddock]

Allows float128 to be used in non gnu mode.
Enable some tests for non-gnu mode.
Fixes #569.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.0%. Comparing base (0ab75f9) to head (9ba6ffd).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #637     +/-   ##
=========================================
+ Coverage     92.5%   94.0%   +1.6%     
=========================================
  Files          254     279     +25     
  Lines        23475   27033   +3558     
=========================================
+ Hits         21699   25396   +3697     
+ Misses        1776    1637    -139     

Files with missing lines	Coverage Δ
config/has_basic_float128.cpp	100.0% <100.0%> (ø)
...nclude/boost/multiprecision/detail/number_base.hpp	97.9% <ø> (ø)

... and 38 files with indirect coverage changes

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