Fix testing of the standard library with Emscripten

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

library/std/tests/pipe_subprocess.rs

@@ -1,7 +1,7 @@
 #![feature(anonymous_pipe)]
 
 fn main() {
-    #[cfg(all(not(miri), any(unix, windows)))]
+    #[cfg(all(not(miri), any(unix, windows), not(target_os = "emscripten")))]
     {
         use std::io::{Read, pipe};
         use std::{env, process};

library/std/tests/process_spawning.rs

@@ -5,7 +5,8 @@ use std::{env, fs, process, str};
 mod common;
 
 #[test]
-#[cfg_attr(any(miri, target_os = "wasi"), ignore)] // Process spawning not supported by Miri and wasi
+// Process spawning not supported by Miri, Emscripten and wasi
+#[cfg_attr(any(miri, target_os = "emscripten", target_os = "wasi"), ignore)]
 fn issue_15149() {
     // If we're the parent, copy our own binary to a new directory.
     let my_path = env::current_exe().unwrap();