Fix testing of the standard library with Emscripten

This does need EMCC_CFLAGS="-s MAXIMUM_MEMORY=2GB" avoid several OOMs.

library/std/src/io/copy/tests.rs

@@ -126,6 +126,7 @@ mod io_benches {
     use crate::io::prelude::*;
 
     #[bench]
+    #[cfg_attr(target_os = "emscripten", ignore)] // no /dev
     fn bench_copy_buf_reader(b: &mut Bencher) {
         let mut file_in = File::open("/dev/zero").expect("opening /dev/zero failed");
         // use dyn to avoid specializations unrelated to readbuf