libc: Fix dtor order in __cxa_thread_atexit

[aokblast]

Summary

The thread_local variable may creates another thread_local variable
inside its dtor. This new object is immediately be registered in
__cxa_thread_atexit() and need to be freed before processing another
variable.

This fixes the libcxx test thread_local_destruction_order.pass.cpp.

[aokblast]

Test case in here.
https://github.com/llvm/llvm-project/blob/5eaf19a151296f770a1a2bd8514a76530234d933/libcxxabi/test/thread_local_destruction_order.pass.cpp

[kib]

lib/libc/stdlib/cxa_thread_atexit_impl.c

125

This is

dtor = LIST_FIRST(&dtors);

But usually it is written as

    while ((dtor = LIST_FIRST(&dtors)) != NULL) {
          LIST_REMOVE();
...

[aokblast]

Style fix

[aokblast]

I think we need to refactor these stuff to dynamic array in the future. It makes no sense to use linked_list if we only need to push and pop from the top.