accel/tcg: Check %halted field in cpu_handle_halt() caller

In cpu_exec(), check CPUState::halted field early before
calling cpu_handle_halt(). The logic is the same but allow
to simplify the async event processing hooks that will be
added in the next commit.

Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20260819145649.23439-4-philmd@oss.qualcomm.com>