avr: add atmega1284 chip support

Not tested on actual hardware, only on simavr. The main motivation for
adding this chip is to be able to run simulated tests using a much
larger memory space (16kB RAM, 128kB flash) without jumping to the XMega
devices that may not be as well supported by LLVM.

src/machine/machine_atmega1284p.go

@@ -0,0 +1,12 @@
+// +build avr,atmega1284p
+
+package machine
+
+import "device/avr"
+
+const irq_USART0_RX = avr.IRQ_USART0_RX
+
+// Return the current CPU frequency in hertz.
+func CPUFrequency() uint32 {
+	return 20000000
+}