[openmp][WebAssembly] Allow openmp to compile and run under emscripten toolchain (#95169)

* Separate wasi and emscripten as they have different constraints and
abilities
* Emscripten mimics Linux/POSIX by statically linking the musl runtime.
This allow nearly all KMP_OS_LINUX code paths to work correctly. There
are only a few places that need to be adjusted related to dynamic
linking (dl_open)
* Internally link openmp globals
* With CommonLinkage it is needed to emit them in an assembly file, now
they are defined and used within each compilation unit
* With ExternalLinkage they suffer from duplicate symbols during linking
for unnamed globals like reduction/critical
   * Interestingly this aligns with the TODO comment above this code

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