cgo: support errno value as second return parameter

Making this work on all targets was interesting but there's now a test
in place to make sure this works on all targets that have the CGo test
enabled (which is almost all targets).

src/runtime/runtime_tinygowasmp2.go

@@ -81,3 +81,9 @@ func procUnpin() {
 func hardwareRand() (n uint64, ok bool) {
 	return random.GetRandomU64(), true
 }
+
+func libc_errno_location() *int32 {
+	// CGo is unavailable, so this function should be unreachable.
+	runtimePanic("runtime: no cgo errno")
+	return nil
+}