Implement CLOCK_MONOTONIC_RAW (linux >= 2.6.28).

It is documented as a raw hardware-based clock not subject to NTP or
incremental adjustments. With this "not as precise as CLOCK_MONOTONIC"
description in mind, map it to our CLOCK_MONOTNIC_FAST (the same
mapping as for the linux CLOCK_MONOTONIC_COARSE).

This is needed for the webcomponent of steam (chromium) and some
other steam component or game.

The linux-steam-utils port contains a LD_PRELOAD based fix for this.
There this is mapped to CLOCK_MONOTONIC.
As an untrained ear/eye (= the majority of people) is normaly not
noticing a difference of jitter in the 10-20 ms range, specially
if you don't pay attention like for example in a browser session
while watching a video stream, the mapping to CLOCK_MONOTONIC_FAST
seems more appropriate than to CLOCK_MONOTONIC.