ftrace: default to tilegx if ARCH=tile is specified

This matches the existing behavior in arch/tile/Makefile for defconfig.

Reported-by:  <fengguang.wu@intel.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Tony Lu <zlu@tilera.com>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>