tile: add clock_gettime support to vDSO

This change adds support for clock_gettime with CLOCK_REALTIME
and CLOCK_MONOTONIC using vDSO.  It also updates the vdso
struct nomenclature used for the clocks to match the x86 code
to keep it easier to update going forward.

We also support the *_COARSE clockid_t, for apps that want speed
but aren't concerned about fine-grained timestamps; this saves
about 20 cycles per call (see http://lwn.net/Articles/342018/

).

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Acked-by: John Stultz <john.stultz@linaro.org>