[x86] Fix CLWB documentation.

[Koval, Julia]

Hi,
This is small fix for documentation - it adds CLWB to skylake-avx512 and 
removes it from cannonlake.

gcc/
        * doc/invoke.texi (Skylake Server): Add CLWB.
        (Cannonlake): Remove CLWB.

Thanks,
Julia

doc_patch_16.2.18
Description: doc_patch_16.2.18