Do not rely on Clang's results to verify our GCC version handling

[tautschnig]

We use __float128 to distinguish GCC versions, which is not a
distinguishing factor for Clang. (It's supported on several Clang
versions on x86, but not on Apple's M1/ARM.) This commit effectively
disables the gcc_version1 test on macOS (where "gcc" really is Clang).

Fixes: #5828

• 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.

[NlightNFotis]

Tested on the M1 platform, and the tests are now passing. Thanks 👍🏻

[codecov]

Codecov Report

Merging #5829 (09251eb) into develop (9778868) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5829   +/-   ##
========================================
  Coverage    72.85%   72.85%           
========================================
  Files         1423     1423           
  Lines       154072   154072           
========================================
  Hits        112244   112244           
  Misses       41828    41828           

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