gallivm: disable NEON instructions if they are not supported

The LLVM project made some questionable decisions about defaults for
armv7 (e.g. they enable NEON that is not there on NVIDIA and Marvell
platforms).

On top of that, getHostCPUFeatures() doesn't disable missing machine
attributes. Finally, -neon alone is not sufficient to disable emmision
of NEON instructions.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit e983a975)