libclc: Fix FP_ILOGBNAN definition

Fix FP_ILOGBNAN definition to match the opencl-c-base.h one and
guarantee that FP_ILOGBNAN and FP_ILOGB0 are different. Doing that
implies fixing ilogb() implementation to return the right value.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>

Reviewed By: jvesely

Differential Revision: https://reviews.llvm.org/D83473