RFR: JDK-8266536: Provide a variant of os::iso8601_time which works with arbitrary timestamps [v3]

[Thomas Stuefe]

> We have os::iso8601_time(), which gives an ISO8601 timestamp of the current time. It would be very useful to have a second variant which can be fed an arbitrary numerical timestamp.
> 
> This is useful in the context of making asynchronous UL logging cheaper (see JDK-8229517)
> 
> This patch provides an additional API: 
> `char* os::iso8601_time(jlong milliseconds_since_19700101, char* buffer, size_t buffer_length, bool utc);`
> alongside the existing
> `char* os::iso8601_time(char* buffer, size_t buffer_length, bool utc);`
> and implements the latter using the former. Not much code added.
> 
> In addition, it adds a regression gtest for these APIs.
> 
> Please ignore the harfbuzz change, its a build fix needed for older gcc, will be removed before final push.
> 
> Testing: GHA, manual gtests, SAP nightlies on all our platforms.
> 
> Thanks, Thomas

Thomas Stuefe has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:

 - Merge
 - remove harfbuzz fix
 - Remove duplicate coding in gtest
 - gtest for os.iso8601_time should use utc
 - Build fix for Harfbuzz
 - Merge
 - start

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3869/files
  - new: https://git.openjdk.java.net/jdk/pull/3869/files/942b4324..40f23a23

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3869&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3869&range=01-02

  Stats: 2297 lines in 100 files changed: 1553 ins; 600 del; 144 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3869.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3869/head:pull/3869

PR: https://git.openjdk.java.net/jdk/pull/3869