target/sh4: Replace TB_FLAG_GUSA_EXCLUSIVE with CF_STEP_ATOMIC

There was a complex bug with gUSA wherein:

  * decode_gusa calls gen_restart_exclusive
  * gen_restart_exclusive generates code that sets TB_FLAG_GUSA_EXCLUSIVE
    and generates a call to helper_exclusive

  * when the code is executed, TB_FLAG_GUSA_EXCLUSIVE is set
  * helper_exclusive calls cpu_loop_exit_atomic, this makes cpu_exec exit
    with EXCP_ATOMIC
  * we go to cpu_loop, we execute cpu_exec_step_atomic
  * suppose that exit request is set, cpu_exec_step_atomic does nothing, it
    leaves the CPU in the same state as it was before
  * we go back to cpu_loop
  * suppose that no signal is delivered, so the gUSA is not rewound
  * cpu_loop goes to cpu_exec
  * there is one difference - now, TB_FLAG_GUSA_EXCLUSIVE is set and it was
    clear before - so cpu_exec will not use the TB that calls
    helper_exclusive, it will instead use the TB that performs the atomic
    operation (both of these TBs have the same PC, they only differ in flags)
  * the TB that performs the atomic operation is executed inside cpu_exec
    => race condition

Fix the bug by managing the "are we in cpu_exec_step_atomic" flag
from cpu_exec_step_atomic itself, rather from the translator.

Cc: qemu-stable@nongnu.org
Fixes: 4bfa602bc22 ("target/sh4: Handle user-space atomics")
Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Helge Deller <deller@gmx.de>
(cherry picked from commit 62728f8c932f9572eeea22f91e3ee223978b1c7f)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>