main: support gdb debugging with AVR

Be able to run `tinygo gdb -target=arduino examples/serial` and debug a
program with the power of a real debugger.

Note that this only works on LLVM 11 because older versions have a bug
in the AVR backend that cause it to produce invalid debug information:
https://reviews.llvm.org/D74213.

targets/avr.json

@@ -15,5 +15,6 @@
 	"extra-files": [
 		"src/internal/task/task_stack_avr.S",
 		"src/runtime/gc_avr.S"
-	]
+	],
+	"gdb": "avr-gdb"
 }