tcg: Initialize return value after exit_atomic

Users of tcg_gen_atomic_cmpxchg and do_atomic_op rightfully utilize
the output.  Even though this code is dead, it gets translated, and
without the initialization we encounter a tcg_error.

Reported-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Tested-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>