Add 'make spell' target, fix what it finds. In .go files, only checks comments.

src/internal/task/task_stack_avr.S

@@ -28,7 +28,7 @@ tinygo_startTask:
     // After return, exit this goroutine. This is a tail call.
 #if __AVR_ARCH__ == 2 || __AVR_ARCH__ == 25
     // Small memory devices (≤8kB flash) that do not have the long call
-    // instruction availble will need to use rcall instead.
+    // instruction available will need to use rcall instead.
     // Note that they will probably not be able to run more than the main
     // goroutine anyway, but this file is compiled for all AVRs so it needs to
     // compile at least.