arch/tile: don't validate CROSS_COMPILE needlessly

With this change, the arch/tile Makefile will only check for a valid
combination of CROSS_COMPILE vs "uname -m" for a few common targets
that are typically the ones we get wrong (vmlinux, all, and modules).
The change handles the case of an empty "make" goal like "make all".

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>