avr: add attiny1616 support

This is just support for the chip, no boards are currently supported.
However, you can use this target on a custom board.

Notes:

  - This required a new runtime and machine implementation, because the
    hardware is actually very different (and much nicer than older
    AVRs!).
  - I had to update gen-device-avr to support this chip. This also
    affects the generated output of other AVRs, but I checked all chips
    we support and there shouldn't be any backwards incompatible
    changes.
  - I did not implement peripherals like UART, I2C, SPI, etc because I
    don't need them. That is left to do in the future.

You can flash these chips with only a UART and a 1kOhm resistor, which
is really nice (no special hardware needed). Here is the program I've
used for this purpose: https://pypi.org/project/pymcuprog/

src/machine/machine_attiny1616.go

@@ -0,0 +1,52 @@
+//go:build attiny1616
+
+package machine
+
+import (
+	"device/avr"
+)
+
+const (
+	portA Pin = iota * 8
+	portB
+	portC
+)
+
+const (
+	PA0 = portA + 0
+	PA1 = portA + 1
+	PA2 = portA + 2
+	PA3 = portA + 3
+	PA4 = portA + 4
+	PA5 = portA + 5
+	PA6 = portA + 6
+	PA7 = portA + 7
+	PB0 = portB + 0
+	PB1 = portB + 1
+	PB2 = portB + 2
+	PB3 = portB + 3
+	PB4 = portB + 4
+	PB5 = portB + 5
+	PB6 = portB + 6
+	PB7 = portB + 7
+	PC0 = portC + 0
+	PC1 = portC + 1
+	PC2 = portC + 2
+	PC3 = portC + 3
+	PC4 = portC + 4
+	PC5 = portC + 5
+	PC6 = portC + 6
+	PC7 = portC + 7
+)
+
+// getPortMask returns the PORT peripheral and mask for the pin.
+func (p Pin) getPortMask() (*avr.PORT_Type, uint8) {
+	switch {
+	case p >= PA0 && p <= PA7: // port A
+		return avr.PORTA, 1 << uint8(p-portA)
+	case p >= PB0 && p <= PB7: // port B
+		return avr.PORTB, 1 << uint8(p-portB)
+	default: // port C
+		return avr.PORTC, 1 << uint8(p-portC)
+	}
+}