Profiling: measure time on non-Windows/POSIX using clock_gettime (wip)

PowerPC CPU profiling was only enabled on Windows (using QueryPerformanceCounter). Try to do the same thing using POSIX clock_gettime.
Could we have a default implementation that returns zero values if posix timers with monolithic clock aren't implemented?
That would remove some annoying ifdefs in other files that are ifdef checking if these functions exist.