Block reading of uart and usb

[sago35]

caution

• This change breaks compatibility with previous versions of TinyGo.
• This change improves compatibility with Go.

---

#2625 allows the use of os.Stdin.
os.Stdin and machine.Serial are the same, but the behavior of the following code is different.
When used as os.Stdin, it blocks when there is no data.
However, if machine.Serial, machine.UART or machine.USB is used directly, it will not block.

// It works as expected.
scanner := bufio.NewScanner(os.Stdin)

// Not working as expected.
scanner := bufio.NewScanner(machine.Serial)

I believe that according to the Go documentation, if there is no data, it should block or return an error.

https://pkg.go.dev/io#Reader
Implementations of Read are discouraged from returning a zero byte count with a nil error, except when len(p) == 0.