Add llvm libcxx package.

[dlorenc]

Fixes:

Related:

Pre-review Checklist

For new package PRs only

• This PR is marked as fixing a pre-existing package request bug
  • Alternatively, the PR is marked as related to a pre-existing package request bug, such as a dependency
• REQUIRED - The package is available under an OSI-approved or FSF-approved license
• REQUIRED - The version of the package is still receiving security updates

For security-related PRs

• The security fix is recorded in the advisories repo

For version bump PRs

• The epoch field is reset to 0
• Patch source: patch source here

[dlorenc]

Ref https://github.com/orgs/wolfi-dev/discussions/4794