Auto merge of #38598 - brson:em, r=alexcrichton

Emscripten targets are Unix targets

I suspect this will fix the errors compiling libc https://buildbot.rust-lang.org/builders/auto-linux-rustbuild-cross-opt/builds/689/steps/compile/logs/stdio that are occurring on every PR.

Emscripten is basically a posix emulation layer for the web and I consider it a Unix.

cc @alexcrichton