Rollup merge of rust-lang#31985 - ashleysommer:libbackgrace_emscripten_fix, r=alexcrichton

Was getting error:
```
running: "sh" "/home/flubba86/rust/src/libstd/../libbacktrace/configure" "--with-pic" "--disable-multilib" "--disable-shared" "--disable-host-shared" "--host=asmjs-unknown-emscripten" "--build=x86_64-unknown-linux-gnu"
...
Invalid configuration `asmjs-unknown-emscripten': system `emscripten' not recognized
```
This commit adds the emscripten target to the libbacktrace configure script so it is no longer unrecognized.

Author: Manishearth

src/libstd/build.rs

@@ -23,7 +23,7 @@ fn main() {
 
     let target = env::var("TARGET").unwrap();
     let host = env::var("HOST").unwrap();
-    if !target.contains("apple") && !target.contains("msvc") {
+    if !target.contains("apple") && !target.contains("msvc") && !target.contains("emscripten"){
         build_libbacktrace(&host, &target);
     }