swr used to build and link the rasterizer to the driver, and to support
multiple architectures we needed to have multiple versions of the
driver/rasterizer combination, which needed to link in much of mesa.

Changing to having one instance of the driver and just building
architecture specific versions of the rasterizer gives a large reduction
in disk space.

libGL.so        6464 Kb ->  7000 Kb
libswrAVX.so   10068 Kb ->  5432 Kb
libswrAVX2.so   9828 Kb ->  5200 Kb

Total          26360 Kb -> 17632 Kb

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>