Reduce code size for AVR (#116)

* all: add support for specifying target CPU in target config
* avr: specify the chip name in the target CPU

This reduces code size by a large margin. For examples/blinky, it
reduces code size from 1360 to 1266 when compiling for the Arduino Uno
(94 bytes, or ~7%).

targets/arduino.json

@@ -1,6 +1,7 @@
 {
 	"inherits": ["avr"],
 	"llvm-target": "avr-atmel-none",
+	"cpu": "atmega328p",
 	"build-tags": ["arduino", "atmega328p", "atmega", "avr5"],
 	"cflags": [
 		"-mmcu=atmega328p"