ci: Remove LLVM from ARM test drivers.

ci: Remove LLVM from ARM test drivers.

The LLVM libraries were a significant fraction of the entire payload
(55M/250M uncompressed) into the initramfs of the test boards, but
LLVM is only used for the draw module used in select/feedback (which
isn't even tested in CI on ARM yet).

Assume that llvmpipe draw is safe enough for ARM given the coverage on
x86, and disable LLVM for these jobs.

Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3928>