MINOR: clock: test all clock_gettime() return values

MINOR: clock: test all clock_gettime() return values

Till now we were only using clock_gettime() for profiling, so if it
would fail it was no big deal. We intend to use it as the main clock
as well now, so we need to more reliably detect its absence or failure
and gracefully fall back to other options. Without the test we would
return anything present in the stack, which is neither clean nor easy
to detect.