As support expands to more runtimes, we'll need to
distinguish between more than just HSA and unknown.
This also lets us stop using unknown everywhere.
Neither am I. Mesa should definitely be the vendor though, not the OS. Michel said on IRC that a mesa vulkan driver will probably end up using a different ABI, so we will probably need to distinguish those.
We've been treating the driver as the operating system. I have no idea what the OS would mean if mesa is an environment, The host OS is irrelevant. I'm still iffy about whether clover qualifies as a separate OS from the graphics driver, since it doesn't really share any of the same interface to kernels despite being another mesa project.