Reimplement clock(3) using clock_gettime(3)

Reimplement clock(3) using clock_gettime(3)

Unlike times(), clock_gettime() is implemented as a vDSO on many architectures.
So, using clock_gettime() will return a more accurate time and do so with less
overhead because it does have the overhead of calling into the kernel.

It is also significantly more accurate because it measures the actual time in
nanoseconds rather than the number of ticks (typically 1 millisecond or more).

Bug: 17814435

(cherry picked from commit 8d0b2dbf2154d5da17ff09b1d4f864d281362ad2)

Change-Id: Id4945d9f387330518f78669809639952e9227ed9