goto-cc/gcc command line: add option coverage

[tautschnig]

GCC supports -coverage to enable coverage instrumentation. For now, we
silently ignore this option to avoid warnings about an unsupported
option. We could, however, consider inserting coverage instrumentation
as --cover does with goto-instrument or cbmc.

• 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 #6537 (88057de) into develop (6642254) will decrease coverage by 0.00%.
The diff coverage is 93.38%.

@@             Coverage Diff             @@
##           develop    #6537      +/-   ##
===========================================
- Coverage    75.99%   75.98%   -0.01%     
===========================================
  Files         1578     1578              
  Lines       180944   181038      +94     
===========================================
+ Hits        137500   137557      +57     
- Misses       43444    43481      +37     

Impacted Files	Coverage Δ
src/goto-cc/gcc_cmdline.cpp	74.80% <ø> (ø)
src/goto-instrument/contracts/assigns.h	100.00% <ø> (ø)
src/goto-instrument/contracts/contracts.h	100.00% <ø> (ø)
src/goto-instrument/contracts/utils.cpp	89.47% <78.57%> (-4.56%)	⬇️
src/goto-instrument/contracts/assigns.cpp	95.83% <90.00%> (-0.60%)	⬇️
src/goto-instrument/contracts/contracts.cpp	91.13% <98.46%> (-3.99%)	⬇️
src/goto-instrument/contracts/utils.h	100.00% <100.00%> (ø)
src/analyses/dirty.h	100.00% <0.00%> (ø)
src/analyses/locals.h	100.00% <0.00%> (+14.28%)	⬆️

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