arch/tile: make glibc's sysconf(_SC_NPROCESSORS_CONF) work correctly

glibc assumes that it can count /sys/devices/system/cpu/cpu* to get
the number of configured cpus.  For this to be valid on tile, we need
to generate a "cpu" entry for all cpus, including the ones that are
not currently allocated for Linux's use.

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