Merge pull request rust-lang#3899 from tgross35/wasi-cherry-pick

[0.2] Add wasm32-wasip2 definitions necessary for std::net support

Author: tgross35

src/wasi/mod.rs

@@ -873,3 +873,10 @@ cfg_if! {
         pub use self::p2::*;
     }
 }
+
+cfg_if! {
+    if #[cfg(target_env = "p2")] {
+        mod p2;
+        pub use self::p2::*;
+    }
+}