Allow building Mesalib with option llvm on arm.
This requires disabling nouveau and radeon drivers.

The performance boost is appreciable, glxgears is twice as fast on
a cubieboard 2:

cubiem:/>/usr/X11R7/bin/glxgears
76 frames in 5.1 seconds = 15.022 FPS
75 frames in 5.0 seconds = 14.889 FPS
75 frames in 5.0 seconds = 14.979 FPS
69 frames in 5.1 seconds = 13.653 FPS
80 frames in 5.1 seconds = 15.831 FPS
78 frames in 5.0 seconds = 15.529 FPS
cubiem:/>/usr/pkg/bin/glxgears
121 frames in 5.0 seconds = 24.076 FPS
184 frames in 5.0 seconds = 36.595 FPS
182 frames in 5.0 seconds = 36.072 FPS
175 frames in 5.0 seconds = 34.718 FPS
171 frames in 5.0 seconds = 34.053 FPS
169 frames in 5.0 seconds = 33.734 FPS

on the beaglebone the boost is nearly 2x too:
beagle:~> /usr/X11R7/bin/glxgears
47 frames in 5.0 seconds =  9.343 FPS
47 frames in 5.1 seconds =  9.246 FPS
47 frames in 5.1 seconds =  9.210 FPS
47 frames in 5.1 seconds =  9.253 FPS
47 frames in 5.1 seconds =  9.252 FPS
47 frames in 5.1 seconds =  9.236 FPS
beagle:~> /usr/pkg/bin/glxgears
2 frames in 7.3 seconds =  0.275 FPS
84 frames in 5.1 seconds = 16.613 FPS
84 frames in 5.0 seconds = 16.641 FPS
83 frames in 5.0 seconds = 16.482 FPS
83 frames in 5.0 seconds = 16.480 FPS
83 frames in 5.0 seconds = 16.436 FPS


(bouyer)