Auto merge of #831 - alexcrichton:wasm, r=malbarbo

Compile an empty library on wasm32 non-Emscripten

In preparation for eventually having a non-Emscripten based wasm32 target, this
commit makes `libc` the crate an empty library on wasm32 targets that are not
with `target_os = "emscripten"`. This may eventually get filled out over time,
but for now it's all empty!