RFR: 8273239: Standardize Ticks APIs return type

Wed Sep 1 15:05:32 UTC 2021

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

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Commit messages:
 - tick-double

Changes: https://git.openjdk.java.net/jdk/pull/5332/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5332&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273239
  Stats: 33 lines in 5 files changed: 0 ins; 0 del; 33 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5332.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5332/head:pull/5332

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