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/avrtiny.json

@@ -0,0 +1,25 @@
+{
+	"llvm-target": "avr",
+	"build-tags": ["avr", "avrtiny", "baremetal", "linux", "arm"],
+	"goos": "linux",
+	"goarch": "arm",
+	"gc": "conservative",
+	"linker": "ld.lld",
+	"scheduler": "none",
+	"rtlib": "compiler-rt",
+	"libc": "picolibc",
+	"default-stack-size": 256,
+	"cflags": [
+		"-Werror"
+	],
+	"ldflags": [
+		"-T", "targets/avrtiny.ld",
+		"--gc-sections"
+	],
+	"extra-files": [
+		"src/internal/task/task_stack_avr.S",
+		"src/runtime/asm_avr.S",
+		"targets/avrtiny.S"
+	],
+	"gdb": ["avr-gdb"]
+}