RFR: 8348387: Add fixpath if needed for user-supplied tools

[Magnus Ihse Bursie]

When discovering tools, configure checks if it is a program that can handle unix-style paths or not. If it isn't, a fixpath prefix is automatically added.

But when the user supplies a tool like `configure FOO=foo.exe`, this does not happen. If that tool does not understand unix-style paths, then it will fail, and there is no easy way for the user to fix this.

Instead, we should apply the fixpath check also to user-supplied tools.

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Commit messages:
 - Add fixpath if needed for user-supplied tools

Changes: https://git.openjdk.org/jdk/pull/23257/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23257&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348387
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/23257.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23257/head:pull/23257

PR: https://git.openjdk.org/jdk/pull/23257