The root of the problem is that ./configure mixed together two different things. One is the
availability of GCC atomic operation intrinsics. The other is the selection of which
primitives to use for my_atomic implementation.

Then at some point a hack was made to not use GCC intrinsics in my_atomic to work around
some test failures. But because the two things are mixed in ./configure, this as a side
effect also makes GCC intrinsics unavailable for XtraDB.

Fixed by splitting this in two in configure, so that we have HAVE_GCC_ATOMIC_BUILTINS for
GCC intrinsics availability and MY_ATOMIC_MODE_GCC_BUILTINS for use in my_atomic.