RFR: 8338851: Hoist os::Posix::realpath() to os::realpath() and implement on Windows [v6]

[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 incrementally with one additional commit since the last revision:

  added gtest for realpath

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20683/files
  - new: https://git.openjdk.org/jdk/pull/20683/files/33c4b402..20f697a6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20683&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20683&range=04-05

  Stats: 44 lines in 1 file changed: 44 ins; 0 del; 0 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