[x86] Fix CLWB documentation.

[Jeff Law]

On 02/16/2018 03:13 AM, Koval, Julia wrote:
> 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.
OK.

jeff