Fix compiling libstd with emscripten target.

Fix compiling libstd with emscripten target.
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
```

Undo change to libbacktrace configure script.
Modify libstd build.rs to not build libbacktrace in the case of targeting emscripten.