wasip2: Invert conditional to include p2 APIs

This commit switches `cfg(target_feature = "p2")` to instead using
`cfg(not(target_feature = "p1"))` to be more future-proof of new Rust
targets such as `wasm32-wasip3`. All future targets will support the
same set of functionality in `wasm32-wasip2`, so this should be valid
for future targets.

(backport #4733)

(cherry picked from commit d2fb5b0)

Author: alexcrichton

src/wasi/mod.rs

@@ -846,7 +846,7 @@ extern "C" {
 }
 
 cfg_if! {
-    if #[cfg(target_env = "p2")] {
+    if #[cfg(not(target_env = "p1"))] {
         mod p2;
         pub use self::p2::*;
     }