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.

Makefile

@@ -287,6 +287,8 @@ smoketest:
 	$(TINYGO) build -size short -o test.hex -target=pca10056-s140v7     examples/blinky1
 	@$(MD5SUM) test.hex
 ifneq ($(AVR), 0)
+	$(TINYGO) build -size short -o test.hex -target=atmega1284p         examples/serial
+	@$(MD5SUM) test.hex
 	$(TINYGO) build -size short -o test.hex -target=arduino             examples/blinky1
 	@$(MD5SUM) test.hex
 	$(TINYGO) build -size short -o test.hex -target=arduino-nano        examples/blinky1

src/machine/machine_atmega.go

@@ -234,7 +234,7 @@ func (uart UART) Configure(config UARTConfig) {
 	}
 
 	// Register the UART interrupt.
-	interrupt.New(avr.IRQ_USART_RX, func(intr interrupt.Interrupt) {
+	interrupt.New(irq_USART0_RX, func(intr interrupt.Interrupt) {
 		// Read register to clear it.
 		data := avr.UDR0.Get()
 

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
+}

src/machine/machine_atmega328p.go

@@ -0,0 +1,7 @@
+// +build avr,atmega328p
+
+package machine
+
+import "device/avr"
+
+const irq_USART0_RX = avr.IRQ_USART_RX

targets/atmega1284p.json

@@ -0,0 +1,19 @@
+{
+    "inherits": ["avr"],
+    "llvm-target": "avr-atmel-none",
+    "cpu": "atmega1284p",
+    "build-tags": ["atmega1284p", "atmega"],
+    "cflags": [
+        "-mmcu=atmega1284p"
+    ],
+    "ldflags": [
+        "-mmcu=avr51",
+        "-Wl,--defsym=_bootloader_size=0",
+        "-Wl,--defsym=_stack_size=512"
+    ],
+    "linkerscript": "src/device/avr/atmega1284p.ld",
+    "extra-files": [
+        "targets/avr.S",
+        "src/device/avr/atmega1284p.s"
+    ]
+}