x86-64: Add _CET_ENDBR to STRCMP_SSE42

Add _CET_ENDBR to STRCMP_SSE42, which is called indirectly, to support
IBT.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

	* sysdeps/x86_64/multiarch/strcmp-sse42.S (STRCMP_SSE42): Add
	_CET_ENDBR.