RFR: 8273239: Standardize Ticks APIs return type

[Kim Barrett]

On Wed, 1 Sep 2021 14:38:52 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Simple change on return types of Ticks API.
> 
> The call of `milliseconds()` in `spinYield.cpp` seems a bug to me, because the unit in the message is `usecs`. Therefore, I changed it to `microseconds()`.
> 
> Test: tier1

Converting nanosecond time values to double can be information losing.  I think some helper functions to provide floating point values could be useful, but I think the existing functions should not be changed as proposed.

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PR: https://git.openjdk.java.net/jdk/pull/5332