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/

tools/gen-device-avr/gen-device-avr.go

@@ -333,6 +333,15 @@ func readATDF(path string) (*Device, error) {
 		}
 	}
 
+	// Flash that is mapped into the data address space (attiny10, attiny1616,
+	// etc).
+	mappedFlashStart := int64(0)
+	for _, name := range []string{"MAPPED_PROGMEM", "MAPPED_FLASH"} {
+		if segment, ok := memorySizes["data"].Segments[name]; ok {
+			mappedFlashStart = segment.start
+		}
+	}
+
 	flashSize, err := strconv.ParseInt(memorySizes["prog"].Size, 0, 32)
 	if err != nil {
 		return nil, err
@@ -379,6 +388,7 @@ func readATDF(path string) (*Device, error) {
 			"flashSize":        int(flashSize),
 			"ramStart":         ramStart,
 			"ramSize":          ramSize,
+			"mappedFlashStart": mappedFlashStart,
 			"numInterrupts":    len(device.Interrupts),
 		},
 		interrupts: interrupts,
@@ -629,6 +639,9 @@ func writeLD(outdir string, device *Device) error {
 /* Generated by gen-device-avr.go from {{.file}}, see {{.descriptorSource}} */
 
 __flash_size = 0x{{printf "%x" .flashSize}};
+{{if .mappedFlashStart -}}
+__mapped_flash_start = 0x{{printf "%x" .mappedFlashStart}};
+{{end -}}
 __ram_start = 0x{{printf "%x" .ramStart}};
 __ram_size   = 0x{{printf "%x" .ramSize}};
 __num_isrs   = {{.numInterrupts}};