os, runtime: enable os.Stdin for baremetal target

src/runtime/runtime_attiny.go

@@ -14,6 +14,16 @@ func putchar(c byte) {
 	// UART is not supported.
 }
 
+func getchar() byte {
+	// UART is not supported.
+	return 0
+}
+
+func buffered() int {
+	// UART is not supported.
+	return 0
+}
+
 func sleepWDT(period uint8) {
 	// TODO: use the watchdog timer instead of a busy loop.
 	for i := 0x45; i != 0; i-- {