xtensa: replace variant/core.h with asm/core.h

Introduce the header arch/xtensa/include/asm/core.h that provides
definitions for XCHAL macros missing in older xtensa releases. Use this
header instead of variant/core.h

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/lib/strnlen_user.S

@@ -12,8 +12,8 @@
  */
 
 #include <linux/linkage.h>
-#include <variant/core.h>
 #include <asm/asmmacro.h>
+#include <asm/core.h>
 
 /*
  * size_t __strnlen_user(const char *s, size_t len)