RFR: 8338851: Hoist os::Posix::realpath() to os::realpath() and implement on Windows [v2]

[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

Simon Tooke has updated the pull request incrementally with two additional commits since the last revision:

 - simplify windwos realpath() implementation
 - get rid of os::posix::realpath() and os::win32::realpath()

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20683/files
  - new: https://git.openjdk.org/jdk/pull/20683/files/e59cad60..b7f495b2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20683&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20683&range=00-01

  Stats: 108 lines in 4 files changed: 36 ins; 68 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