anv, radv: OpenBSD build fixes

Jonathan Gray requested to merge jsg/mesa:vulkan-openbsd into master

Some patches to make anv and radv build on OpenBSD. Mostly deals with using common code in util to get memory sizes. radv needed a different patch in 20.1 to avoid seccomp, since 7324977e that is mostly gone apart from some remaining includes which can be removed.

Merge request reports