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/

targets/attiny1616.json

@@ -0,0 +1,14 @@
+{
+    "inherits": ["avrtiny"],
+    "cpu": "attiny1616",
+    "build-tags": ["attiny1616"],
+    "gc": "none",
+    "cflags": [
+        "-D__AVR_ARCH__=103"
+    ],
+    "linkerscript": "src/device/avr/attiny1616.ld",
+    "extra-files": [
+        "src/device/avr/attiny1616.s"
+    ],
+    "flash-command": "pymcuprog write -f {hex} --erase --verify -d attiny1616 -t uart -u {port}"
+}