RFR: 8316532: Native library copying in BuildMicrobenchmark.gmk cause dups on macOS

[Erik Joelsson]

After [JDK-8253620](https://bugs.openjdk.org/browse/JDK-8253620), BuildMicrobenchmark.gmk has started printing warnings about overriding targets on macos. This is caused by a CopyFiles using the `FLATTEN` option in combination with debug symbols on macos having the same filename as the dylib itself.

In this fix, I'm changing the CopyFiles to no longer flatten the file structure. I'm also filtering the list of files to be copied to only include files from the native output `lib` dir. This avoids having any files from the support dir (e.g. dependency *.d files) from being copied into the test image. The resulting native directory in the test-image ends up having all native libraries in it, with debug symbols in their respective *.dSYM sub-directories, as expected.

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Commit messages:
 - JDK-8316532

Changes: https://git.openjdk.org/jdk/pull/15824/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15824&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316532
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/15824.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15824/head:pull/15824

PR: https://git.openjdk.org/jdk/pull/15824