Fix tests on wasm

The `std` test straightforwardly can't work without file descriptors;
 #99417 tracks moving it out of tests/ui.

`issue-13560.rs` requires the target to support dynamic linking.

`extern-mod-syntax` is interesting. The original test was added to check
if `extern mod` could be parsed correctly and used `extern mod std` and
an import:
138dc3048a (diff-73700e1e851b7a37bc92174635dab726124c82e5bfabbbc45b4a3c2e8e14fadd)
At some point `std::json::Object` was moved out of std to an unstable
rustc-only `extras` crate, and rather than just changing the import it
got changed to use the unstable crate. When `extras` was removed, people
assumed the test was meant to also test rustc_private and changed it to
another unstable crate rather than using something in std.

This changes the test to remove the `rustc_private` import, to allow it
to work properly when cross-compiling.

tests/ui/extern-mod-syntax.rs

@@ -1,11 +1,11 @@
 // run-pass
 
 #![allow(unused_imports)]
-#![feature(rustc_private)]
+#![no_std]
 
-extern crate libc;
-use libc::c_void;
+extern crate std;
+use std::ffi::c_void;
 
 pub fn main() {
-    println!("Hello world!");
+    std::println!("Hello world!");
 }

tests/ui/issue-13560.rs

@@ -1,4 +1,5 @@
 // run-pass
+// ignore-cross-compile (needs dylibs and compiletest doesn't have a more specific header)
 // aux-build:issue-13560-1.rs
 // aux-build:issue-13560-2.rs
 // aux-build:issue-13560-3.rs

tests/ui/std/switch-stdout.rs

@@ -1,4 +1,5 @@
 // run-pass
+// ignore-wasm (needs file descriptors and env variables)
 
 use std::env;
 use std::fs::File;