RFR: JDK-8266536: Provide a variant of os::iso8601_time which works with arbitrary timestamps [v4]

[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 incrementally with one additional commit since the last revision:

  feedback yasumasa and xin

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3869/files
  - new: https://git.openjdk.java.net/jdk/pull/3869/files/40f23a23..e6461c58

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3869&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3869&range=02-03

  Stats: 9 lines in 4 files changed: 3 ins; 0 del; 6 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