Summary:
This patch implements `clock_gettime` using the monotonic clock. This
allows users to get time elapsed at nanosecond resolution. This is
primarily to facilitate compiling the `chrono` library from `libc++`.
For this reason we provide both `CLOCK_MONOTONIC`, which we can
implement
with the GPU's global fixed-frequency clock, and `CLOCK_REALTIME` which
we cannot. The latter is provided just to make people who use this
header happy and it will always return failure.
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