clover/llvm: undefine __IMAGE_SUPPORT__ for devices without image support

clover/llvm: undefine __IMAGE_SUPPORT__ for devices without image support

libclang seems to define this on its own for SPIR targets, but the CTS
requires it to be not set if the device doesn't support images.

The SPIRV-LLVM-Translator also requires the spir triple to be set so we
can't really do anything else except to undefine.

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Jason Ekstrand <jason@jlekstran.net>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6433>