rust: time: Avoid 64-bit integer division on 32-bit architectures

Avoid 64-bit integer division that 32-bit architectures don't
implement generally. This uses ktime_to_us() and ktime_to_ms()
instead.

The time abstraction needs i64 / u32 division so C's div_s64() can be
used but ktime_to_us() and ktime_to_ms() provide a simpler solution
for this time abstraction problem on 32-bit architectures.

32-bit ARM is the only 32-bit architecture currently supported by
Rust. Using the cfg attribute, only 32-bit architectures will call
ktime_to_us() and ktime_to_ms(), while the other 64-bit architectures
will continue to use the current code as-is to avoid the overhead.

One downside of calling the C's functions is that the as_micros/millis
methods can no longer be const fn. We stick with the simpler approach
unless there's a compelling need for a const fn.

Suggested-by: Arnd Bergmann <[email protected]>
Suggested-by: Boqun Feng <[email protected]>
Signed-off-by: FUJITA Tomonori <[email protected]>
Reviewed-by: Andreas Hindborg <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Andreas Hindborg <[email protected]>

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