fix unit testing for gcc-11++

[westram]

Code generated by newer gccs (starting with gcc-11-series) does no longer work with arbs unit tests.

Required changes:

• use objdump instead of nm to parse symbols
• use own format for generated symlists
• fix detection of symbol locations
• repair duplicate detector
• add separate warning level to
  • suppress tests marked as broken, but
  • show all other warnings

Test that fixed unit tester works with all gcc series from 5 to 15.

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related to #873