Fix __float128 portability

[tautschnig]

GCC does not provide this type on all architectures.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Patch coverage: 20.00% and project coverage change: +0.45% 🎉

Comparison is base (f40203a) 78.05% compared to head (afb6b2e) 78.50%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7843      +/-   ##
===========================================
+ Coverage    78.05%   78.50%   +0.45%     
===========================================
  Files         1698     1698              
  Lines       195117   195119       +2     
===========================================
+ Hits        152289   153176     +887     
+ Misses       42828    41943     -885     

Files Changed	Coverage Δ
src/cpp/cpp_internal_additions.cpp	86.04% <20.00%> (-2.05%)	⬇️

... and 56 files with indirect coverage changes

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[TGWDB]

Looks like there may be some more combinations of architecture and types we're not testing in regression tests, but approving on the basis of incremental improvement is better than waiting for perfect test coverage.