Add ARM-without-NEON mode for Halide

A patch to allow generating code for ARMv7 that doesn't have NEON; this
is mainly to allow for experimenting on Tegra 2 chips, which have this
unfortunate limitation.

I'm not sure if this patch should actually land, as performance is
about as poor on this architecture as you might expect; I wouldn't mind
a once-over from someone more familiar with ARM assembly, though, as I
may have been overzealous in disabling instructions (there may be
instructions that are still legal on VFPv3-D16 that aren't getting
emitted here.)