Initial OpenMP support

[JAicewizard]

This makes emcc recognize -fopenmp, and links it as needed.

When updating openmp, it neesd to be compiled. This is not strictly neccesary, but we need to invoke cmake in order to generate the headers. When compiling openemp, we need to make use of cmake as well, as the build system is quite long and complicated. I am sure it can be done another way using ninja, and I encourage others to implement this, but I do not have the capacity right now to make this happen.

Fixes: #13892
Fixes: #17637

[JAicewizard]

To clarify, I have tested this by compiling + linking a program requiring openmp

[sbc100]

I assume the contents of system/lib/libomp is coming from an upstream repo?

Can you using git submodule for this and put it in third_party/ maybe?

[JAicewizard]

I assume the contents of system/lib/libomp is coming from an upstream repo?

Can you using git submodule for this and put it in third_party/ maybe?

Yess, it is from llvm. Since all the other llvm libraries are in this location, it made sense to put it here. Do you want the entire LLVM repository as a submodule? It seems a bit overkill, at that point the other libraries can also be pulled from there

[sbc100]

I assume the contents of system/lib/libomp is coming from an upstream repo?
Can you using git submodule for this and put it in third_party/ maybe?

Yess, it is from llvm. Since all the other llvm libraries are in this location, it made sense to put it here. Do you want the entire LLVM repository as a submodule? It seems a bit overkill, at that point the other libraries can also be pulled from there

Oh i see, I didn't realize this was from llvm. I that case the import approach your took does seem reasonable.

[JAicewizard]

Failing tests:

• other.test_shell_cmd_with_quotes: No idea, the failing command works for me, but it fails in the test. No idea how to debug
• wasm2js1.test_openmp_max_threads: Seems to fail because of unimplemented in binaryen: Assertion !curr->isAtomic && "atomic store not implemented"' failed.`
• wasm64_4gb.test_openmp_max_threads: I fixed that libomp is no longer compiled for 32bit on 64 bit target, however libomp is not supported for 64bit wasm.

For the last two I would like to disable the tests, but I don't know how.
For the first, I really don't know how to find out what the issue is, let alone how to fix it.

Ruff issues can be easily fixed ofc