runtime: handle GODEBUG on wasip2 (#5312)

* runtime: handle GODEBUG on wasip2
* Support env on more platforms to get GODEBUG working

src/runtime/env_nonhosted.go

@@ -0,0 +1,43 @@
+//go:build baremetal || js || wasm_unknown
+
+package runtime
+
+//go:linkname syscallSetenv syscall.runtimeSetenv
+func syscallSetenv(key, value string) {
+	entry := key + "=" + value
+	for i, e := range env {
+		if envKey(e) == key {
+			env[i] = entry
+			if key == "GODEBUG" && godebugUpdate != nil {
+				godebugUpdate(key, value)
+			}
+			return
+		}
+	}
+	env = append(env, entry)
+	if key == "GODEBUG" && godebugUpdate != nil {
+		godebugUpdate(key, value)
+	}
+}
+
+//go:linkname syscallUnsetenv syscall.runtimeUnsetenv
+func syscallUnsetenv(key string) {
+	for i, e := range env {
+		if envKey(e) == key {
+			env = append(env[:i], env[i+1:]...)
+			if key == "GODEBUG" && godebugUpdate != nil {
+				godebugUpdate(key, "")
+			}
+			return
+		}
+	}
+}
+
+func envKey(entry string) string {
+	for i := 0; i < len(entry); i++ {
+		if entry[i] == '=' {
+			return entry[:i]
+		}
+	}
+	return entry
+}