target/ppc: do not reset reserve_addr in exec_enter

target/ppc: do not reset reserve_addr in exec_enter

In case when atomic operation is not supported, exit_atomic is called
and we stop the world and execute the atomic operation. This results
in a following call chain:

tcg_gen_atomic_cmpxchg_tl()
  -> gen_helper_exit_atomic()
     -> HELPER(exit_atomic)
        -> cpu_loop_exit_atomic() -> EXCP_ATOMIC
           -> qemu_tcg_cpu_thread_fn() => case EXCP_ATOMIC
              -> cpu_exec_step_atomic()
                 -> cpu_step_atomic()
                    -> cc->cpu_exec_enter() = ppc_cpu_exec_enter()
                       Sets env->reserve_addr = -1;

But by the time it return back, the reservation is erased and the code
fails, this continues forever and the lock is never taken.

Instead set this in powerpc_excp()

Now that ppc_cpu_exec_enter() doesn't have anything meaningful to do,
let us get rid of the function.

Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>