runtime: use clock_gettime instead of gettimeofday on Darwin

[randall77]

clock_gettime has higher resolution, we should use it if available.
It is not guaranteed to be available until Darwin 10.12. This issue is on hold until that is our minimum Darwin version.

We could do this now but it would require making the choice at runtime. That seems difficult - we'd need to use the Darwin equivalent of dlopen/dlsym to query the existence of, and call, clock_gettime.

[gopherbot]

Change https://golang.org/cl/161721 mentions this issue: unix: add ClockGettime on darwin/amd64

[randall77]

Go 1.13 requires Darwin 1.11 or newer. Getting closer...

[randall77]

Go 1.15 will require 10.12. Setting that milestone.

[gopherbot]

Change https://golang.org/cl/270918 mentions this issue: runtime: use clock_gettime instead of gettimeofday on darwin

[bradfitz]

@tklauser, any positive performance improvements as a result of this? I seem to recall another bug where getting the time on macOS was really slow for some reason.

[tklauser]

@bradfitz I didn't see any performance improvements. The benchmarks seem more or less unchanged (macOS 10.15 on 1.6 GHz Dual-Core Intel Core i5):

name           old time/op  new time/op  delta
Now-4          83.3ns ± 4%  80.9ns ± 2%  -2.81%  (p=0.009 n=10+10)
NowUnixNano-4  85.3ns ± 4%  85.1ns ± 3%    ~     (p=1.000 n=10+10)

Results seems to be a bit inconsistent though, for some runs I also got slightly increased numbers (around +2%).