runtime: implement environment variables for Linux

src/runtime/runtime.go

@@ -89,8 +89,3 @@ func AdjustTimeOffset(offset int64) {
 func os_sigpipe() {
 	runtimePanic("too many writes on closed pipe")
 }
-
-//go:linkname syscall_runtime_envs syscall.runtime_envs
-func syscall_runtime_envs() []string {
-	return nil
-}