RFR: 8338851: Hoist os::Posix::realpath() to os::realpath() and implement on Windows [v15]

[Simon Tooke]

> This PR changes the status of realpath() from a Posix-specific API to a globally available API, i.e. adding it to the "Hotspot Porting API".  Code would refer to os::realpath() instead of os::Posix::realpath().
> 
> This requires a Windows implementation of realpath(), using Windows _fullpath(), and renaming os::Posix::realpath() to os::realpath().
> 
> The main difference between POSIX and Windows behaviour is that POSIX actually requires an existing accessible file, while Windows will happily work with made-up filenames.
> 
> Please note that guidelines for doing this appear in src/hotspot/share/runtime/os.hpp

Simon Tooke has updated the pull request incrementally with one additional commit since the last revision:

  account for Windows behviour changes

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20683/files
  - new: https://git.openjdk.org/jdk/pull/20683/files/6715569a..e1b2c534

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20683&range=14
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20683&range=13-14

  Stats: 8 lines in 2 files changed: 4 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/20683.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20683/head:pull/20683

PR: https://git.openjdk.org/jdk/pull/20683