kernel: extend time unit conversion to high resolution scales

Some use cases require using high-resolution tick or cycle clocks to
measure sub-millisecond durations.  Generate the corresponding 32-bit
conversions to avoid the cost of 64-bit math in the common case where
the duration fits in 32 bits in both original and converted scale.

Signed-off-by: Peter Bigot <[email protected]>

Author: pabigot