linux-user/main.c: Call cpu_exec_start/end on all target archs

The start_exclusive() infrastructure is used on all target
architectures, even if only to do the "stop all CPUs before
dumping core" in force_sig(), so be consistent and call
cpu_exec_start/end in the main loop of every target.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>