Add optimization plain option

The optimization built-in option now accepts plain value, which will not set any optimization flags. This is now the default value of the flag for buildtype=plain, which is useful for distros, that set the optimization and hardening flags by other means.

If you are using the value of get_option('optimization') in your Meson scripts, make sure you are not making assumptions about it, such as that the value can be passed to a compiler in -O flag.