llvm: Added back libcxx libcxxabi libunwind runtime projects

[smuzaffar]

No description provided.

[smuzaffar]

please test

[cmsbuild]

A new Pull Request was created by @smuzaffar for branch IB/CMSSW_16_1_X/master.

@akritkbehera, @iarspider, @raoatifshad, @smuzaffar can you please review it and eventually sign? Thanks.
@ftenchini, @mandrenguyen, @sextonkennedy you are the release manager for this.
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Failed Tests: UnitTests
Summary: https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-3a520f/52238/summary.html
COMMIT: b675bd9
CMSSW: CMSSW_16_1_X_2026-03-24-2300/el8_amd64_gcc13
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week0/cms-sw/cmsdist/10445/52238/install.sh to create a dev area with all the needed externals and cmssw changes.

The following merge commits were also included on top of IB + this PR after doing git cms-merge-topic:

• @cms-sw git-lfs: set GO variable to point to cms externals #10438
• @cms-sw llvm: Drop libcxx, libcxxabi, libunwind runtime as we use gcc-toolchain #10442

You can see more details here:
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-3a520f/52238/git-recent-commits.json
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-3a520f/52238/git-merge-result

Failed Unit Tests

I found 1 errors in the following unit tests:

---> test TestDQMOnlineClient-gem_dqm_sourceclient had ERRORS

Comparison Summary

Summary:

• You potentially added 16 lines to the logs
• ROOTFileChecks: Some differences in event products or their sizes found
• Reco comparison results: 10 differences found in the comparisons
• DQMHistoTests: Total files compared: 53
• DQMHistoTests: Total histograms compared: 4185300
• DQMHistoTests: Total failures: 6
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 4185274
• DQMHistoTests: Total skipped: 20
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.0 KiB( 52 files compared)
• Checked 227 log files, 198 edm output root files, 53 DQM output files
• TriggerResults: no differences found