RFR: 8338851: Hoist os::Posix::realpath() to os::realpath() and implement on Windows [v21]

[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:

  clean up comments per PR review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20683/files
  - new: https://git.openjdk.org/jdk/pull/20683/files/b7e6043f..fbafbf07

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20683&range=20
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20683&range=19-20

  Stats: 14 lines in 1 file changed: 1 ins; 0 del; 13 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