RFR: 8314491: Linux: jexec launched via PATH fails to find java

[Vladimir Petko]

8314491: Linux: jexec launched via PATH fails to find java

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Commit messages:
 - Use /proc/self/exec to identify path to the executable.
 - Create failing test for jexec in PATH issue

Changes: https://git.openjdk.org/jdk/pull/15343/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15343&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314491
  Stats: 46 lines in 2 files changed: 29 ins; 9 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/15343.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15343/head:pull/15343

PR: https://git.openjdk.org/jdk/pull/15343