Disable check for __float128 on PGI compiler

[mborland]

Fixes #640

Per config we should not have 128-bit float on this platform: https://github.com/boostorg/config/blob/19701e05d4832d1e97e06e791f2e4e1c7cba9b13/include/boost/config/compiler/pgi.hpp#L19.

Not sure if anything has happened in the 7 years since that commit was last updated...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.1%. Comparing base (0ab75f9) to head (5108a96).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #641     +/-   ##
=========================================
+ Coverage     92.5%   94.1%   +1.6%     
=========================================
  Files          254     279     +25     
  Lines        23475   28885   +5410     
=========================================
+ Hits         21699   27161   +5462     
+ Misses        1776    1724     -52     

Files with missing lines	Coverage Δ
...nclude/boost/multiprecision/detail/number_base.hpp	98.0% <ø> (+0.2%)	⬆️

... and 165 files with indirect coverage changes

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[mborland]

The 3 failed run in circle all provide 0 information as to the failure, it's all build killed. Since GitHub actions and drone both show no problems it's likely safe to merge this. The issue reporter also says it fixed their problem.