linux-user/s390x: Fix single-stepping SVC

Currently single-stepping SVC executes two instructions. The reason is
that EXCP_DEBUG for the SVC instruction itself is masked by EXCP_SVC.
Fix by re-raising EXCP_DEBUG.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20230510230213.330134-2-iii@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
(cherry picked from commit 01b9990)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

linux-user/s390x/cpu_loop.c

@@ -86,6 +86,15 @@ void cpu_loop(CPUS390XState *env)
             } else if (ret != -QEMU_ESIGRETURN) {
                 env->regs[2] = ret;
             }
+
+            if (unlikely(cs->singlestep_enabled)) {
+                /*
+                 * cpu_tb_exec() did not raise EXCP_DEBUG, because it has seen
+                 * that EXCP_SVC was already pending.
+                 */
+                cs->exception_index = EXCP_DEBUG;
+            }
+
             break;
 
         case EXCP_DEBUG: