Auto merge of #1478 - tlively:update-emscripten-types, r=gnzlbg

Update Emscripten system types

These changes bring the types up to parity with recent Emscripten
versions using the upstream LLVM wasm backend. These changes should be
coordinated with the upgrade of rustc's Emscripten support. See
https://internals.rust-lang.org/t/upgrading-rust-s-emscripten-support/10684