[PATCH] [libcxx] Partially address a FIXME in steady_clock::now()

[Jon Roelofs]

mclow and I discussed this offline, and max= is not going to work:

20.12.7.2/1: Objects of class steady_clock represent clocks for which values of time_point never decrease as physical time advances and for which **values of time_point advance at a steady rate relative to real time**.

I'll look into what breaks if steady_clock isn't provided on systems without _POSIX_MONOTONIC_CLOCK, and go from there.

http://reviews.llvm.org/D4045