internal/task: rename tinygo_pause to tinygo_task_exit

This is more descriptive: the call is to exit a task, not to pause it.
This also makes it more obvious that there's an optimization
opportunity: to free the stack explicitly after the goroutine returns
(or to keep it as a cache for the next stack allocation).

src/internal/task/task_stack_avr.S

@@ -32,10 +32,10 @@ tinygo_startTask:
     // 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.
-    rcall  tinygo_pause
+    rcall  tinygo_task_exit
 #else
     // Other devices can (and must) use the regular call instruction.
-    call tinygo_pause
+    call tinygo_task_exit
 #endif
 
 .global tinygo_swapTask