HAL_Linux: fixed time function to use integer maths

[tridge]

avoid floating point rounding errors after long uptimes.

thanks to Richard (RSAXVC) for finding this

[Williangalvani]

Started testing:
root@blueos:/# uptime
19:28:10 up 4:07, 1 user, load average: 4.59, 3.92, 3.12
root@blueos:/#

I'll edit this and attach a log in 1h

https://drive.google.com/file/d/1dEm8MidI1MUHxzNZCH7egOP7eRSDZh1j/view?usp=drive_link

[rsaxvc]

Without this patch I saw the same sort of issues running the Scheduler_test, so I ran it under a few different conditions:

32-bit float conversion, 3-week OS uptime, after about a minute of ardupilot uptime, without this patch the scheduler is running the one_hz at around 2Hz :

five_seconds: t=53375 ins_counter=3750
one_hz: t=54000
one_hz: t=54500
one_hz: t=55000
one_hz: t=55500
one_hz: t=56000
five_seconds: t=56000 ins_counter=4000

Same as above, with this patch(uint64_t nanoseconds), good scheduling is restored:

five_seconds: t=55907 ins_counter=2750
one_hz: t=56907
one_hz: t=57907
one_hz: t=58907
one_hz: t=59907
one_hz: t=60907
five_seconds: t=60907 ins_counter=3000

[tridge]

@Williangalvani looks good