tile: default to tilegx_defconfig for ARCH=tile

There is no "ARCH=tile" (just like there is no "ARCH=x86") so we need
to pick a default configuration, either tilepro or tilegx, when users
specify ARCH=tile.  We'll use tilegx, since that's our current chip.

Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>