feather-m0: UART0 not exported

The Feather M0 board file ([`board_feather-m0.go`](https://github.com/tinygo-org/tinygo/blob/release/src/machine/board_feather-m0.go)) does not export UART0 nor the TX/RX pins for UART0. The exported pins `UART_TX_PIN` and `UART_RX_PIN` are to UART1.