bug#36786: Warn of AMD GPUs unusable with Guix System

[Denis 'GNUtoo' Carikli]

Would it be better to find a way to fix these issue rather than warn
people about it? They look realatively easy to fix.

For the Radeon / AMDGPU drivers to work, you need:
- A patch need to be upstreamed in linux-libre for the GPU family to
  make the driver load. That patch needs to be tested on real hardware.
- Potentially some configuration for Xorg for recent cards to account
  for the lack of hardware 2D acceleration. Without it we could have
  something like that (from Parabola):
  '/usr/lib/xorg/modules/drivers/radeon_drv.so: undefined symbol:
  exaGetPixmapDriverPrivate'

The Libreplanet wiki[1] has all the information about that. If each
person with an unsupported GPU and a bit of skills (it requires to
compile linux-libre) and a bit of time (it should not take that long,
it'll probably be something between less than 1 hour to 1 day), or that
we try to help people with these GPU doing that, we could fix it for
good.

I already did it for all the AMD GPUs I had access to, but despite the
fact that it's really a low hanging fruit, only 1 person that needed a
lot of help tried to follow these tutorials (though I didn't manage to
find the time it required to help that person until the end).

All the information about was added in the libreplanet wiki[1] as it is
relevant to most GNU/Linux FSDG compliant distributions and the problem
is not specific to a given distribution.

References:
-----------
[1]https://libreplanet.org/wiki/Group:Hardware/research/gpu/radeon

Denis.

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