Draft: main/mesa: stop building softpipe

Draft, unless wants to get this stuff before official release, thou should be kept open, since the configuration change from swrast to llvmpipe to get benefit of not shipping softpipe.

When falling back to software rendering, llvmpipe is used (as always been). From functional POV nothing changes.

Saves around 133k of text on aarch64.

Signed-off-by: David Heidelberg david@ixit.cz