Move (or copy ?) set_sys_clock_khz() from stdlib to clocks.h

[MikeDB1]

I would like to not include stdlib but for some reason set_sys_clock_khz() is only present in it. Moving or copying this function that most people need to clocks.h would allow stdlib not to be included. Kilograham agreed this would be a good idea.

[kilograham]

yes, this code is definitely in the wrong place