llvm: correctly lower c-abi for Wasm target

When lowering the return type for Wasm if the calling convention is `C`,
it now correctly lower it according to what clang does as specified in:
https://github.com/WebAssembly/tool-conventions/blob/main/BasicCABI.md

This makes use of the same logic as the Wasm backend, ensuring the
generated code does not diverge in function signatures.

When passing arguments accross the C-ABI for the Wasm target,
we want slightly different behavior than x86_64.
For instance: a struct with multiple fields must always be passed
by reference, even if its ABI size fits in a single integer.
However, we do pass larger integers such as 128bit by value,
which LLVM will correctly lower to use double arguments instead.

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