I checked man pages for clock_gettime on a system running Red Hat EL 3 (quite an old distro nowadays) and did not find CLOCK_REALTIME_HR there; neiherI found it referenced somewhere in /usr/include. Little googling gaveme a number of references to a kernel.org maillist posting saying that CLOCK_REALTIME_HR was removed a while ago.
There is no direct connection between tbb::tick_count and the actual tick count. As you correctly noticed, we rely on OS services to obtain a timer with good enough resolution. In case someone knows a better solution for a particular OS, it's the chance to contribute to TBB :)
We deliberately use OS services instead of HW counters for getting time stamps. There are several reasons for that, and making it work correctly in the parallel environment called by multiple threads is amongst those. Also, on some older processors HW tick count follows CPU frequency, thus is affected by dynamic frequency change (not sure if OSes fix it, though). And, TBB is more portable relying on OS services than if it would use HW directly.
So if anyone knows better (more precise, more reliable, more portable, etc) OS timing service than those used now in TBB, welcome to suggest it or contribute code.