use qemu_cpu_kick instead of cpu_exit or qemu_cpu_kick_thread

Use the same API to trigger interruption of a CPU, no matter if
under TCG or KVM.  There is no difference: these calls come from
the CPU thread, so the qemu_cpu_kick calls will send a signal
to the running thread and it will be processed synchronously,
just like a call to cpu_exit.  The only difference is in the
overhead, but neither call to cpu_exit (now qemu_cpu_kick)
is in a hot path.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

gdbstub.c

@@ -1362,7 +1362,7 @@ void gdb_do_syscall(gdb_syscall_complete_cb cb, const char *fmt, ...)
        is still in the running state, which can cause packets to be dropped
        and state transition 'T' packets to be sent while the syscall is still
        being processed.  */
-    cpu_exit(s->c_cpu);
+    qemu_cpu_kick(s->c_cpu);
 #endif
 }