RFR: 8338851: Hoist os::Posix::realpath() to os::realpath() and implement on Windows [v10]

[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 the addition of a stub routine in os_posix.cpp and a Windows implementation of realpath(), using Windows _fullpath().
> 
> This PR depends on #20597 in that it removes the need for one #ifdef in that PR.  Because of that, this PR will be modified when and if #20597 is integrated (or vice-versa)
> 
> Please note that guidelines for doing this appear in src/hotspot/share/runtime/os.hpp

Simon Tooke 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 15 additional commits since the last revision:

 - remove conditional compilation
 - Merge branch 'master' into pr_windows_realpath
 - fix realpath() test for POSIX
 - Define MAX_PATH if required
 - use MAX_PATH only
 - added gtest for realpath
 - remove empty line
 - fix indentation
 - fix missing return statement
 - properly test for buffer too small for path
 - ... and 5 more: https://git.openjdk.org/jdk/compare/2269f18b...15db6b75

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20683/files
  - new: https://git.openjdk.org/jdk/pull/20683/files/ee870f7c..15db6b75

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20683&range=09
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20683&range=08-09

  Stats: 173592 lines in 1548 files changed: 156846 ins; 8724 del; 8022 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