Make test identifier_names_illegal_characters work on Windows

[tautschnig]

GCC built-ins are only available with a GCC-like front-end, and thus
need to be wrapped in #ifdef GNUC. While at it, also reduce
duplicate infrastructure by moving the test to the goto-instrument
folder.

• 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

Merging #5571 (d97fe4c) into develop (aa069d3) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5571   +/-   ##
========================================
  Coverage    69.31%   69.31%           
========================================
  Files         1241     1241           
  Lines       100407   100407           
========================================
+ Hits         69596    69601    +5     
+ Misses       30811    30806    -5     

Flag	Coverage Δ
cproversmt2	43.08% <ø> (ø)
regression	66.22% <ø> (+<0.01%)	⬆️
unit	32.27% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/goto-instrument/goto_program2code.cpp	66.40% <0.00%> (+0.64%)	⬆️

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