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

src/librustc_back/target/asmjs_unknown_emscripten.rs

@@ -23,6 +23,7 @@ pub fn target() -> Result<Target, String> {
         obj_is_bitcode: true,
         max_atomic_width: Some(32),
         post_link_args: vec!["-s".to_string(), "ERROR_ON_UNDEFINED_SYMBOLS=1".to_string()],
+        target_family: Some("unix".to_string()),
         .. Default::default()
     };
     Ok(Target {

src/librustc_back/target/wasm32_unknown_emscripten.rs

@@ -26,6 +26,7 @@ pub fn target() -> Result<Target, String> {
         max_atomic_width: Some(32),
         post_link_args: vec!["-s".to_string(), "BINARYEN=1".to_string(),
                              "-s".to_string(), "ERROR_ON_UNDEFINED_SYMBOLS=1".to_string()],
+        target_family: Some("unix".to_string()),
         .. Default::default()
     };
     Ok(Target {