[NativeAOT-LLVM] Add a native WASM object writer

[SingleAccretion]

This adds an object writer that directly emits WASM object files (see https://github.com/WebAssembly/tool-conventions/blob/main/Linking.md) and uses it to emit data symbols. It is anticipated that future changes will use it for code (compiler-generated helpers) as well.

We have also decided to delete the LLVM-based object writer, since doing otherwise would imply either maintaining it (duplication of effort) or letting it bitrot. It means that ILC will no longer produce all output as LLVM bitcode.

There are two motivations for this change:

1. (Primary) [NativeAOT-LLVM] Implement managed stack traces #2404 - we need to be able to emit relocations LLVM doesn't support.
2. (Secondary) Performance: the new object writer will, of course, skip all of the LLVM steps, which have been observed to be rather lengthy on larger projects due to some algorithmic issues in LLVM.

[yowl]

This looks great!

[SingleAccretion]

@dotnet/nativeaot-llvm

[yowl]

This looks good to me. I'm not familiar with the object file layout, so I'm going by the fact that as it works, it is correct. Do we have any before/after numbers for total wasm size ?

[SingleAccretion]

Do we have any before/after numbers for total wasm size ?

Yes - this is a no-diff change, as expected.

[jkotas]

Thank you!