[jdk11u-dev] RFR: 8273153: Consolidate file_exists into os:file_exists

[Elif Aslan]

This is not a clean backport. There was a merge conflict on os.cpp due to diverged os::split_path method for this reason  os::file_exists method was manually added.
This backport is required to address a crash observed in Amazon production.

tier2 tests were run  in addition to GHA

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Commit messages:
 - Backport of 9732fbe428c3b6a5422cc94e7295ba5482d1a7a9

Changes: https://git.openjdk.org/jdk11u-dev/pull/2721/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2721&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8273153
  Stats: 26 lines in 4 files changed: 9 ins; 13 del; 4 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2721.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2721/head:pull/2721

PR: https://git.openjdk.org/jdk11u-dev/pull/2721