* libc/include/sys/config.h (_REENT_GLOBAL_ATEXIT): Define for
RTEMS.
* libc/include/sys/reent.h (_reent): Use _REENT_GLOBAL_ATEXIT.
(_global_atexit): Declare if _REENT_GLOBAL_ATEXIT is defined.
* libc/reent/reent.c (_reclaim_reent): Remove atexit cleanup if
_REENT_GLOBAL_ATEXIT is defined.
(_wrapup_reent): Remove atexit handling if _REENT_GLOBAL_ATEXIT
is defined.
* libc/stdlib/__atexit.c (_global_atexit0): Define if
_REENT_GLOBAL_ATEXIT is defined.
* libc/stdlib/__call_atexit.c (_global_atexit): Define if
_REENT_GLOBAL_ATEXIT is defined.