kern_time: Honor the precise option when counting diff

[aokblast]

Summary

When preecise option is used, the true elapsed time should also use the
precise timer.

This fixes the test case sleep_for.signals.pass.cpp in libcxx.

[aokblast]

https://github.com/llvm/llvm-project/blob/788b2bde7b31615fd22c906b1acf2ba2072acf6b/libcxx/test/std/thread/thread.threads/thread.thread.this/sleep_for.signals.pass.cpp

The test is in here, which failed originally since the diff time is not >= 500ms.

[aokblast]

Formatting

[imp]

I read through everything two different times, and convinced myself both times this is the proper change.