Categorize WASI as an "OS" rather than as an "environment".

This distinction is fairly abstract, but in practice, the main advantage
here is that LLVM's triple code considers WASI to be an OS, so this
makes rustc agree with that.

src/librustc_target/spec/wasm32_unknown_wasi.rs

@@ -102,8 +102,8 @@ pub fn target() -> Result<Target, String> {
         target_endian: "little".to_string(),
         target_pointer_width: "32".to_string(),
         target_c_int_width: "32".to_string(),
-        target_os: "unknown".to_string(),
-        target_env: "wasi".to_string(),
+        target_os: "wasi".to_string(),
+        target_env: String::new(),
         target_vendor: "unknown".to_string(),
         data_layout: "e-m:e-p:32:32-i64:64-n32:64-S128".to_string(),
         arch: "wasm32".to_string(),

src/libstd/os/mod.rs

@@ -51,7 +51,7 @@ cfg_if! {
 #[cfg(target_os = "emscripten")] pub mod emscripten;
 #[cfg(target_os = "fuchsia")]    pub mod fuchsia;
 #[cfg(target_os = "hermit")]     pub mod hermit;
-#[cfg(target_env = "wasi")]      pub mod wasi;
+#[cfg(target_os = "wasi")]       pub mod wasi;
 #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] pub mod fortanix_sgx;
 
 pub mod raw;

src/libstd/sys/mod.rs

@@ -35,7 +35,7 @@ cfg_if! {
     } else if #[cfg(target_os = "redox")] {
         mod redox;
         pub use self::redox::*;
-    } else if #[cfg(target_env = "wasi")] {
+    } else if #[cfg(target_os = "wasi")] {
         mod wasi;
         pub use self::wasi::*;
     } else if #[cfg(target_arch = "wasm32")] {