Fix build on POWER10

[mborland]

Since this platform has an unsupported long double configuration we need to delete those constructors and conversion functions. Also fix the tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.4%. Comparing base (0874cc8) to head (af807eb).
⚠️ Report is 13 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1086     +/-   ##
=========================================
- Coverage     99.4%   99.4%   -0.0%     
=========================================
  Files          244     244             
  Lines        16743   16743             
  Branches      1812    1812             
=========================================
- Hits         16637   16636      -1     
- Misses         106     107      +1     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	98.5% <ø> (ø)
include/boost/decimal/decimal32_t.hpp	96.5% <ø> (ø)
include/boost/decimal/decimal64_t.hpp	97.2% <ø> (ø)
include/boost/decimal/decimal_fast128_t.hpp	98.8% <ø> (ø)
include/boost/decimal/decimal_fast32_t.hpp	100.0% <ø> (ø)
include/boost/decimal/decimal_fast64_t.hpp	100.0% <ø> (ø)
...clude/boost/decimal/detail/ryu/ryu_generic_128.hpp	100.0% <ø> (ø)
test/github_issue_426.cpp	100.0% <ø> (ø)
test/roundtrip_decimal128.cpp	100.0% <ø> (ø)
test/roundtrip_decimal128_fast.cpp	100.0% <ø> (ø)
... and 15 more

... and 1 file with indirect coverage changes

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