swr: build driver proper separate from rasterizer

swr: build driver proper separate from rasterizer

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>