Do not error on undefined external linker symbols

[karkhaz]

goto-cc currently parses the linker script given by the -T flag to find addresses for variables that are extern-declared in C files but not given a definition. This is because some variables are defined in the linker script, not in C code, and reading these definitions from the linker script is necessary to verify the C program.

Prior to this commit, goto-cc would error out if some extern-defined variables remained undefined even after reading the linker script. There is actually a valid use case for having undefined symbols even after linking, see this (search for "Undefined symbol to cause link failure"):

https://lore.kernel.org/all/[email protected]/T/

This commit turns the error into a warning and synthesizes a null pointer for such symbols.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[harmim]

I would wrap the commit message body at 72 characters. It would be more consistent with other CBMC's commits.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.01 ⚠️

Comparison is base (9f894ae) 78.51% compared to head (b40be48) 78.50%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7621      +/-   ##
===========================================
- Coverage    78.51%   78.50%   -0.01%     
===========================================
  Files         1671     1671              
  Lines       191845   191847       +2     
===========================================
  Hits        150618   150618              
- Misses       41227    41229       +2     

Impacted Files	Coverage Δ
src/goto-cc/linker_script_merge.cpp	78.53% <0.00%> (-0.39%)	⬇️
src/goto-cc/linker_script_merge.h	100.00% <ø> (ø)

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