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.

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"
+    ]
+}