libstd fails to build on emscripten

[Thinkofname]

https://github.com/rust-lang/rust/blob/master/src/libstd/os/mod.rs#L38
The linux os module is named emscripten when targeting javascript but as of aa23c98 the linux module references itself directly https://github.com/rust-lang/rust/blob/master/src/libstd/os/linux/fs.rs#L19 which fails when its been renamed to emscripten

[gifnksm]

I believe #31986 fixes this.

[alexcrichton]

Thanks @gifnksm