RFR: 8257973: UTIL_LOOKUP_PROGS should only find executable files

[Magnus Ihse Bursie]

It turns out that UTIL_LOOKUP_PROGS does not verify that when it find a file in the PATH that matches the given tool name, it accept it straight away. Even if the file is a directory, or is not executable.

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Commit messages:
 - 8257973: UTIL_LOOKUP_PROGS should only find executable files

Changes: https://git.openjdk.java.net/jdk/pull/1715/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1715&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257973
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1715.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1715/head:pull/1715

PR: https://git.openjdk.java.net/jdk/pull/1715