[Bug target/114968] 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

LIU Hao <lh_mouse at 126 dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |i686-w64-mingw32,
                   |                            |x86_64-w64-mingw32
                 CC|                            |nathan at acm dot org
      Known to work|                            |13.1.0
      Known to fail|                            |14.1.0

--- Comment #1 from LIU Hao <lh_mouse at 126 dot com> ---
CC'ing the author of eaeaad3fcac4d7a30b5a256410cb59fa1a3fa9dd.

The declaration of `__cxa_atexit` probably misses the exact attribute, and
requires a same fix.