* Serial teams now use a stack (similar to dispatch buffers)
* Serial teams always use `t_task_team[0]` as the task team and the
second pointer is a next pointer for the stack
`t_task_team[1]` is interpreted as a stack of task teams where each
level is a nested level
```
inner serial team outer serial team
[ t_task_team[0] ] -> (task_team) [ t_task_team[0] ] -> (task_team)
[ next ] ----------------> [ next ] -> ...
```
* Remove the task state memo stack from thread structure.
* Instead of a thread-private stack, use team structure to store
th_task_state of the primary thread. When coming out of a parallel,
restore the primary thread's task state. The new field in the team
structure doesn't cause sizeof(team) to change and is in the cache line
which is only read/written by the primary thread.
Fixes: #50602Fixes: #69368Fixes: #69733Fixes: #79416
41ca9104ac