Fix Aircraft::get_wall_time_us() to use monotonic clock on non-Windows

[m-sasha]

This fixes Clock drift when running ArduPilot SITL in a Docker container on a Mac
The bug is that get_wall_time_us uses gettimeofday, which is not guaranteed to be monotonic.
The fix is to use clock_gettime(CLOCK_MONOTONIC, ...) instead.

Should I perhaps also rename get_wall_time_us to get_system_time_us, as it doesn't actually return wall-clock time (and hasn't returned it on Windows even before this PR)?

[tridge]

this looks like a good change, thanks, just need to ensure it passes CI

[tridge]

btw, the commit message should start with "SITL: ", per our style guide

[m-sasha]

It failed in travis-ci, but it seems the failure wasn't related to my commit:

clang: fatal error: unknown argument: '-mno-thumb-interwork'

[peterbarker]

@m-sasha did you mean to PR this against the Copter stable branch?

Looks like CI is kinda broken there; trying to use a non-cross-compiling clang++ to compile for fmuv3 is ... unintended?

[m-sasha]

I'm not sure which branch it should be merged it into, to be honest. Feel free to choose the right one :-)

[tridge]

closing and replaced by #14487