Fix etl::chrono floor/ceil/round/abs being declared in the wrong namespace
The duration overloads of floor, ceil, round and abs were defined in
namespace etl rather than etl::chrono, so qualified calls such as
etl::chrono::floor<seconds>(ms) only found the time_point overload and
failed to compile. Moved them into namespace chrono next to
duration_cast and removed the old etl-scope definitions, which also
fixes the unqualified calls made from time_point.h.
Fix comment about rounding behavior
Use cmath functions from std namespace
GCC 16 doesn't support them in the global namespace anymore.
ebd9a7026f