avr: add support for tasks scheduler

This adds support for the `-scheduler=tasks` flag for AVR. On most AVR
chips you wouldn't want to run a real scheduler but it may be useful in
some cases, especially on devices with more RAM. It is disabled by
default.

targets/avr.json

@@ -9,5 +9,8 @@
 	"ldflags": [
 		"-T", "targets/avr.ld",
 		"-Wl,--gc-sections"
+	],
+	"extra-files": [
+		"src/runtime/scheduler_avr.S"
 	]
 }