rustc_target: Make sure that in-tree targets follow conventions for os and vendor values

compiler/rustc_target/src/spec/tests/tests_impl.rs

@@ -36,5 +36,18 @@ impl Target {
                 && self.post_link_objects_fallback.is_empty())
                 || self.crt_objects_fallback.is_some()
         );
+        // Keep the default "unknown" vendor instead.
+        assert_ne!(self.vendor, "");
+        if !self.can_use_os_unknown() {
+            // Keep the default "none" for bare metal targets instead.
+            assert_ne!(self.os, "unknown");
+        }
+    }
+
+    // Add your target to the whitelist if it has `std` library
+    // and you certainly want "unknown" for the OS name.
+    fn can_use_os_unknown(&self) -> bool {
+        self.llvm_target == "wasm32-unknown-unknown"
+            || (self.env == "sgx" && self.vendor == "fortanix")
     }
 }