[libc] Display unit test runtime for hosted environments

With more tests added to LLVM libc each week we want to keep track of unittest's runtime, especially for low end build bots.

Top offender can be tracked with a bit of scripting (spoiler alert, mem function sweep tests are in the top ones)
```
ninja check-libc | grep "ms)" | awk '{print $(NF-1),$0}' | sort -nr | cut -f2- -d' '
```

Unfortunately this doesn't work for hermetic tests since `clock` is unavailable.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D151097

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