ins8250.cpp: Implement Break functionality

[mgarlanger]

Compared behavior between a real H19 and the emulated H19, while holding down the break key. Both behaved identically, other keys pressed while the break key was pressed queued up and processed after the break key was released.

[cuavas]

This won’t work properly:

• Setting the break bit in the LCR should immediately pull down TxD, whether data transmission is in progress or not.
• Clearing the break bit in the LCR should immediately set TxD back to whatever was last shifted out of the transmit shift register.

You need to:

• Leave m_txd containing whatever was shifted out of the transmit shift register.
• Call m_out_tx_cb (or device_serial_interface::rx_w for loopback cases) immediately when the break bit changes on a register write.
• Check the break bit before calls to m_out_tx_cb (and device_serial_interface::rx_w for loopback cases) so you don’t change the output when it’s being overridden by the break bit.