[OpenMP][Build][Wasm][116552] Fixed build problem when compiling with Emscripten on Windows

[maniatic0]

Fixed OpenMP build issue when building Wasm using Emscripten in Windows, as raised in #116552, with the proposed solution at #116552 (comment)

Adds the --target-system-override flag to message-converter.py to handle the case where the target system is not the host system. Useful when compiling with Emscripten in Windows, as Python detects the host system as the target system by default when it should be Emscripten.

Sets the use of the previous functionality in \openmp\runtime\src\CMakeLists.txt when compiling with Emscripten to solve the issue raised at #116552

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[github-actions]

✅ With the latest revision this PR passed the Python code formatter.

[shiltian]

The changes look reasonable to me.

[maniatic0]

Thanks! As I don't have write access, could I ask for this PR to be merged?

[github-actions]

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