This prunes out all targets except libgl-gdi, libgl-xlib, and svga, as
suggested by Marek Olšák.

libgl-xlib will be remove once I have had time to confirm no automated
tests we have rely upon it.

There are also a bunch of Makefile.sources which become orphaned as
result, that are not taken care of in this change.

v2: Prune remainders of swr support.

Tested-by: Marge Bot <mesa/mesa!4348>
Part-of: <mesa/mesa!4348>