gallivm: fix alignment issue for vertex data fetch

gallivm: fix alignment issue for vertex data fetch

We cannot guarantee that vertex buffers have the necessary alignment for
fetching all AoS members at once (for instance 4x32bit XYZW data). We can
however guarantee that for textures. This did not cause errors for older
llvm versions but it now matters and will cause segfaults if the data
happens to not be aligned. Thus we need to set alignment manually.
(Note that we can't actually really guarantee data to be even element aligned
due to offsets in vertex buffers being bytes and OpenGL allowing this, but
it does not matter for x86 as alignment is only required for sse vectors -
not sure what happens on other archs, however.)

This fixes https://bugs.freedesktop.org/show_bug.cgi?id=85467.