cpus: Remove CPUClass::has_work() handler

cpus: Remove CPUClass::has_work() handler

All handlers have been converted to SysemuCPUOps::has_work().
Remove CPUClass::has_work along with cpu_common_has_work() and
simplify cpu_has_work(), making SysemuCPUOps::has_work handler
mandatory.

Note, since cpu-common.c is in meson's common_ss[] source set, we
must define cpu_exec_class_post_init() in cpu-target.c (which is
in the specific_ss[] source set) to have CONFIG_USER_ONLY defined.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20250125170125.32855-25-philmd@linaro.org>