arch/tile: enable single-step support for TILE-Gx

This is not quite the complete support, since we're not yet shipping
intvec_64.S, but it is the support relevant to the set of files we are
currently shipping, and makes it easier to track changes between
our internal sources and our public GIT repository.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>