[openmp][wasm] Allow compiling OpenMP to WebAssembly (#71297)

This change allows building the static OpenMP runtime, `libomp.a`, as
WebAssembly. It builds on the work done in [D142593] but goes further in
several ways:
 - it makes the OpenMP CMake files more WebAssembly-aware
- it conditions much more code (or code that had been refactored since
[D142593]) for `KMP_ARCH_WASM` and `KMP_OS_WASI`
- it fixes a Clang crash due to unimplemented common symbols in
WebAssembly

The commit messages have more details. Please understand this PR as a
start, not the completed work, for WebAssembly support in OpenMP.
Getting the tests running somehow would be a good next step, e.g.; but
what is contained here works, at least with recent versions of
[wasi-sdk] and engines that support [wasi-threads]. I suspect the same
is true for Emscripten and browsers, but I have not tested that
workflow.

[D142593]: https://reviews.llvm.org/D142593
[wasi-sdk]: https://github.com/WebAssembly/wasi-sdk
[wasi-threads]: https://github.com/WebAssembly/wasi-threads

---------

Co-authored-by: Atanas Atanasov <atanas.atanasov@intel.com>

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