Auto merge of #31653 - tomaka:emscripten-abi, r=eddyb

Auto merge of #31653 - tomaka:emscripten-abi, r=eddyb

Needs a correct review because I'm not too confident with how this works.
All tests related to the C ABI are now passing.

References:
- https://github.com/kripken/emscripten-fastcomp-clang/blob/dbe68fecd03d6f646bd075963c3cc0e7130e5767/lib/CodeGen/TargetInfo.cpp#L479-L489
- https://github.com/kripken/emscripten-fastcomp-clang/blob/dbe68fecd03d6f646bd075963c3cc0e7130e5767/lib/CodeGen/TargetInfo.cpp#L466-L477

The `classifyArgumentType` function has two different paths depending on `RAA == CGCXXABI::RAA_DirectInMemory`, but I don't really know what's the corresponding option in Rust.

cc @brson @eddyb