clc: undefine spirv defs to work around LLVMs headers

clc: undefine spirv defs to work around LLVMs headers

Clang unconditionally adds those definitions if using a spirv LLVM target.

That's not a problem on its own, but clang's internal OpenCL header enable
a bunch of OpenCL extensions if those are set.

Lucky for us, we can simply undefine them and spare us the trouble of
finding an upstream solution to this problem :)

This fixes the OpenCL CTS' compiler features_macro test.

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17334>