arch/tile: add the <arch> headers to the set of installed kernel headers

These headers are similar to the <asm> headers that describe kernel
APIs, but instead describe aspects of the actual hardware in an
OS- and application-independent manner.  We need to include them in
the set of installed headers so that userspace tools (including glibc)
can build purely from the provided kernel headers.

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