cpus: Build cpu_exec_[un]realizefn() methods once

cpus: Build cpu_exec_[un]realizefn() methods once

Now that cpu_exec_realizefn() and cpu_exec_unrealizefn()
methods don't use any target specific definition anymore,
we can move them to cpu-common.c to be able to build them
once.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250123234415.59850-21-philmd@linaro.org>