Update -mtune=knl for Knights Landing

Update -mtune=knl for Knights Landing

From:

https://software.intel.com/sites/default/files/managed/e9/b5/Knights-Corner-is-your-path-to-Knights-Landing.pdf

Knights Landing is “Based on Intel Atom core (based on Silvermont
microarchitecture) with many HPC enhancements.”

This patch replaces CPU_KNL with CPU_SLM to tune for Knights Landing.

	* config/i386/i386.c (processor_alias_table): Replace CPU_KNL
	with CPU_SLM.
	* config/i386/i386.md (cpu): Remove knl.

From-SVN: r226765