Use sync:1️⃣:Once to fetch the mach_timebase_info only once when
running precise_time_ns(). This helps because mach_timebase_info() is
surprisingly inefficient. Also fix the order of operations when applying
the timebase to the mach absolute time value.
This improves the time on my machine from
```
test tests::bench_precise_time_ns ... bench: 157 ns/iter (+/- 4)
```
to
```
test tests::bench_precise_time_ns ... bench: 38 ns/iter (+/- 3)
```
and it will get even faster once #14174 lands.
8ef3e22719