[Bug target/114968] [14/15 Regression] missing `__thiscall` attribute on builtin declaration of `__cxa_thread_atexit()`

[lh_mouse at 126 dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114968

--- Comment #14 from LIU Hao <lh_mouse at 126 dot com> ---
I suspect it's because that `get_atexit_fn_ptr_type` is shared by `atexit` and
`__cxa_thread_atexit` but the destructors for them do not use the same calling
convention ..

So I should make a copy of it, as well as `atexit_fn_ptr_type_node`.