cpu: Change cpu_single_step() argument to CPUState

Use CPUState::env_ptr for now.

Needed for GdbState::c_cpu.

Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/exec.c b/exec.c
index 30b676d..9cd936c 100644
--- a/exec.c
+++ b/exec.c
@@ -585,10 +585,10 @@
 
 /* enable or disable single step mode. EXCP_DEBUG is returned by the
    CPU loop after each instruction */
-void cpu_single_step(CPUArchState *env, int enabled)
+void cpu_single_step(CPUState *cpu, int enabled)
 {
 #if defined(TARGET_HAS_ICE)
-    CPUState *cpu = ENV_GET_CPU(env);
+    CPUArchState *env = cpu->env_ptr;
 
     if (cpu->singlestep_enabled != enabled) {
         cpu->singlestep_enabled = enabled;