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 1 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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