Added micros64 and used to fix and improve gettimeofday.

[BrandonLWhite]

This resolves #3814 and adds a micros64() global function that is very useful.

There is certainly more that can be done to improve wallclock time-keeping, but this is a step in the right direction I feel. This allows my application to have a reasonably decent current timestamp.

This now works well enough for me, when it previously was wrong:

auto now_ms = std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::system_clock::now().time_since_epoch()).count();

I purposely did not touch clock_gettime() or time() because I want to focus on achieving a functional gettimeofday first. If this PR is accepted, I'm willing to work on those functions too.

[BrandonLWhite]

Thank you! I'm thrilled to have an accepted PR in your project!