RFR: 8273153: Consolidate file_exists into os:file_exists

[Yumin Qi]

This are several places to check if file exists using local similar function, file_exists. They should be consolidated into one version. The patch added a function os::file_exists to do so.

Tests: tier1

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Commit messages:
 - 8273153: Consolidate file_exists into os:file_exists

Changes: https://git.openjdk.java.net/jdk/pull/5308/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5308&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273153
  Stats: 26 lines in 4 files changed: 9 ins; 13 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5308.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5308/head:pull/5308

PR: https://git.openjdk.java.net/jdk/pull/5308