Enable cling based emscripten build

[mcbarton]

Description

Please include a summary of changes, motivation and context for this PR.

If you build roots llvm 20 against cling master with the changes in root-project/root#20231, then you can cross compile a CppInterOp which makes use of cling to wasm using Emscripten. CppInterOps Emscripten tests can build, but the tests fail immediately due to a popen error here in cling https://github.com/root-project/cling/blob/903295516f19014a0097bfd516cb23295ef1a404/lib/Interpreter/CIFactory.cpp#L160

Fixes # (issue)

Type of change

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• Bug fix
• New feature
• Requires documentation updates

Testing

Please describe the test(s) that you added and ran to verify your changes.

Checklist

• I have read the contribution guide recently

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.39%. Comparing base (5f25366) to head (e2b68c7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #739      +/-   ##
==========================================
- Coverage   79.83%   79.39%   -0.45%     
==========================================
  Files           9        9              
  Lines        3963     3780     -183     
==========================================
- Hits         3164     3001     -163     
+ Misses        799      779      -20     

see 8 files with indirect coverage changes
see 8 files with indirect coverage changes

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