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_tinygoriscv.S

@@ -19,7 +19,7 @@ tinygo_startTask:
     jalr  s0
 
     // After return, exit this goroutine. This is a tail call.
-    tail  tinygo_pause
+    tail  tinygo_task_exit
 
 .section .text.tinygo_swapTask
 .global  tinygo_swapTask