Add basic support for running decimal64_t on CUDA

[mborland]

Closes: #1355

[codecov]

Codecov Report

❌ Patch coverage is 97.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.6%. Comparing base (3c4e219) to head (0bcb79b).
⚠️ Report is 29 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/detail/fenv_rounding.hpp	33.4%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1359     +/-   ##
=========================================
+ Coverage     98.6%   98.6%   +0.1%     
=========================================
  Files          282     282             
  Lines        22539   22540      +1     
  Branches      1963    1963             
=========================================
+ Hits         22206   22207      +1     
  Misses         333     333             

Files with missing lines	Coverage Δ
include/boost/decimal/decimal64_t.hpp	94.8% <100.0%> (ø)
include/boost/decimal/detail/attributes.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/div_impl.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/mul_impl.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/utilities.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/fenv_rounding.hpp	97.9% <33.4%> (-1.0%)	⬇️

... and 1 file with indirect coverage changes

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