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

[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.

Please note that guidelines for doing this appear in src/hotspot/share/runtime/os.hpp

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Commit messages:
 - hoist os::Posix::realpath() to os::realpath()

Changes: https://git.openjdk.org/jdk/pull/20683/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20683&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338851
  Stats: 73 lines in 11 files changed: 54 ins; 7 del; 12 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