Add macOS long double to fp_traits for double precision

[mborland]

Closes: #1282

Changes the traits to show that macOS long double on ARM64 is an alias for double. Removes what seems to be an improper fall through case because this allowed a definition of <long double, double_precision> and <long double, extended_precsion> of 128-bits of this platform which is the source of the linked issue error.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.07%. Comparing base (f0a1c2e) to head (83d7b1a).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1283   +/-   ##
========================================
  Coverage    95.07%   95.07%           
========================================
  Files          792      792           
  Lines        66882    66882           
========================================
  Hits         63589    63589           
  Misses        3293     3293           

Files with missing lines	Coverage Δ
.../boost/math/special_functions/detail/fp_traits.hpp	100.00% <ø> (ø)

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