avr: support tinygo run with simavr

This commit adds support for running both the Arduino Uno and the
DigiSpark under a simulator.

targets/atmega328p.json

@@ -10,5 +10,6 @@
 	"extra-files": [
 		"targets/avr.S",
 		"src/device/avr/atmega328p.s"
-	]
+	],
+	"emulator": ["simavr", "-m", "atmega328p", "-f", "16000000"]
 }

targets/digispark.json

@@ -15,5 +15,6 @@
 		"targets/avr.S",
 		"src/device/avr/attiny85.s"
 	],
-	"flash-command": "micronucleus --run {hex}"
+	"flash-command": "micronucleus --run {hex}",
+	"emulator": ["simavr", "-m", "attiny85", "-f", "16000000"]
 }