Use the `x86_64-unknown-none` built-in target to make distro packaging easier

[cclaudio]

@cuviper and @berrange mentioned in the discussion below that it is highly desirable to avoid application specific build targets. From a distro packaging perspective, it would be better to get rid of the current target json file (svsm-target.json) and use the x86_64-unknown-none built-in target.

https://github.com/AMDESE/linux-svsm/pull/35#discussion_r1106025709