Don't require -Wextra goto-gcc flag for warnings to be treated as errors

[danpoe]

• 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 #5393 into develop will decrease coverage by 0.84%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #5393      +/-   ##
===========================================
- Coverage    68.19%   67.34%   -0.85%     
===========================================
  Files         1175     1175              
  Lines        97497    97496       -1     
===========================================
- Hits         66484    65658     -826     
- Misses       31013    31838     +825     

Flag	Coverage Δ
#cproversmt2	?
#regression	65.36% <ø> (-0.01%)	⬇️
#unit	32.24% <ø> (ø)

Impacted Files	Coverage Δ
src/goto-cc/gcc_mode.cpp	67.52% <ø> (-0.10%)	⬇️
src/solvers/smt2/smt2irep.cpp	32.00% <0.00%> (-56.00%)	⬇️
src/solvers/smt2/smt2_conv.cpp	31.46% <0.00%> (-26.23%)	⬇️
src/solvers/floatbv/float_bv.cpp	55.25% <0.00%> (-18.88%)	⬇️
src/solvers/smt2/smt2_dec.cpp	61.53% <0.00%> (-16.93%)	⬇️
src/solvers/smt2/smt2_tokenizer.cpp	69.02% <0.00%> (-16.82%)	⬇️
src/solvers/flattening/pointer_logic.cpp	81.81% <0.00%> (-10.61%)	⬇️
src/solvers/smt2/smt2_parser.cpp	58.91% <0.00%> (-8.07%)	⬇️
src/util/expr_util.cpp	80.50% <0.00%> (-4.24%)	⬇️
src/solvers/smt2/smt2_format.cpp	70.37% <0.00%> (-3.71%)	⬇️
... and 11 more

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