No longer use the ``gcc -fprofile-update=atomic`` flag on i686. The flag has
been added to fix a random GCC internal error on PGO build (:gh:`145801`)
caused by corruption of profile data (.gcda files). The problem is that it
makes the PGO build way slower (up to 47x slower) on i686. Since the GCC
internal error was not seen on i686 so far, don't use
``-fprofile-update=atomic`` on i686 anymore. Patch by Victor Stinner.
