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>
14 jobs for !518 with lr/neon in 17 minutes and 40 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #254315
debian

00:01:55

 
  Build+Test
passed #254317
meson-clang

00:11:47

passed #254323
meson-gallium-clover-llvm

00:06:20

passed #254324
meson-gallium-clover-llvm39

00:04:57

passed #254322
meson-gallium-drivers-other

00:07:16

passed #254321
meson-gallium-radeonsi

00:13:36

passed #254325
meson-gallium-st-other

00:04:40

passed #254316
meson-gallium-swr

00:09:51

passed #254320
meson-glvnd

00:07:50

passed #254319
meson-loader-classic-dri

00:05:40

passed #254318
meson-vulkan

00:03:44

passed #254327
scons-llvm

00:02:34

passed #254326
scons-nollvm

00:07:01

passed #254328
scons-swr

00:06:10