Fix querying emcc on windows

[guusw]

Pretty minor change, just queries emcc.bat since emcc will not work on windows

For example this simple script will output this on windows

fn main() {
    let output = std::process::Command::new("emcc.bat").arg("-dumpversion").output().ok();
    println!("{:?}", output);
    let output1 = std::process::Command::new("emcc").arg("-dumpversion").output().ok();
    println!("{:?}", output1);
}

rustc test.rs && ./test.exe
Some(Output { status: ExitStatus(ExitStatus(0)), stdout: "3.1.74\r\n", stderr: "" })
None

[rustbot]

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[tgross35]

Nit: could you use if cfg!(target_os = "windows") { ... } here instead? cfg!(...) is slightly preferred over #[cfg(...)] where possible since it allows the code to be checked on all OSs.

Otherwise lgtm, please squash with the change.

Fyi @hoodmane.