darwin: invoke `mach_timebase_info` only once

According to @aktau, the call to `mach_timebase_info` costs 90% of the
total execution time of `uv_hrtime()`. The result of the call is static
on all existing platforms, so there is no need in invoking it multiple
times.
That's not entirely thread-safe, not even with the x86's relaxed memory model because loads can still be reordered with stores to different locations.  While some CPU models implement stronger guarantees than that, I wouldn't rely on it.  Remember that ACCESS_ONCE is just a compiler barrier, not a memory barrier.