Support -target in goto-cc GCC/Clang mode

[tautschnig]

Accept and parse Clang's -target
(and --target=-) command-line option.

Fixes: #6906

• 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 #6908 (40859c8) into develop (840c4ec) will decrease coverage by 0.05%.
The diff coverage is 50.11%.

@@             Coverage Diff             @@
##           develop    #6908      +/-   ##
===========================================
- Coverage    77.82%   77.76%   -0.06%     
===========================================
  Files         1568     1568              
  Lines       180004   180318     +314     
===========================================
+ Hits        140092   140231     +139     
- Misses       39912    40087     +175     

Impacted Files	Coverage Δ
src/goto-cc/gcc_cmdline.cpp	79.23% <ø> (+3.84%)	⬆️
src/solvers/flattening/boolbv.h	62.50% <ø> (ø)
src/util/bitvector_expr.cpp	58.55% <0.00%> (-35.65%)	⬇️
src/util/simplify_expr_class.h	90.47% <ø> (ø)
src/solvers/flattening/boolbv_overflow.cpp	43.94% <35.65%> (-4.74%)	⬇️
src/util/simplify_expr.cpp	83.08% <51.37%> (-2.66%)	⬇️
src/solvers/smt2/smt2_conv.cpp	67.39% <57.14%> (-0.22%)	⬇️
src/goto-programs/goto_convert_side_effect.cpp	95.13% <94.59%> (-0.41%)	⬇️
src/util/bitvector_expr.h	97.24% <95.65%> (-0.10%)	⬇️
src/goto-cc/gcc_mode.cpp	68.47% <100.00%> (+0.20%)	⬆️
... and 9 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 16240af...40859c8. Read the comment docs.