Add clock_gettime model

[tautschnig]

Add a library model for clock_gettime:

• Model follows POSIX specification with proper error handling
• Non-deterministic behavior covers both success and failure paths
• Validates timespec structure with appropriate constraints
• Sets errno correctly (EFAULT for null pointer, EINVAL on failure)

Fixes: #7639

• 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.
• 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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.94%. Comparing base (8640fc9) to head (839387a).
⚠️ Report is 9 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8731      +/-   ##
===========================================
+ Coverage    79.93%   79.94%   +0.01%     
===========================================
  Files         1698     1699       +1     
  Lines       187698   187790      +92     
  Branches        73       73              
===========================================
+ Hits        150034   150132      +98     
+ Misses       37664    37658       -6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[kroening]

I'd rename clock_gettime-01/test.desc to something like clock_gettime/test1.desc -- we need to reduce the number of directories.

[tautschnig]

I'd rename clock_gettime-01/test.desc to something like clock_gettime/test1.desc -- we need to reduce the number of directories.

Ack, but I'd like to do this in a different PR - I'll have to check the maintenance scripts nicely cope with this. I'll create such a PR - if it goes in first I'll update this PR accordingly.